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Chapter 2

General Structure

This chapter describes the fundamental principles governing the design of the Unicode Standard and presents an informal overview of its main features. The chapter starts by placing the Unicode Standard in an architectural context by discussing the nature of text representation and text processing and its bearing on character encoding decisions. Next, the Unicode Design Principles are introduced—ten basic principles that convey the essence of the standard. The Unicode Design Principles serve as a tutorial framework for understanding the Unicode Standard.

The chapter then moves on to the Unicode character encoding model, introducing the concepts of character, code point, and encoding forms, and diagramming the relationships between them. This provides an explanation of the encoding forms UTF-8, UTF-16, and UTF-32 and some general guidelines regarding the circumstances under which one form would be preferable to another.

The sections on Unicode allocation then describe the overall structure of the Unicode codespace, showing a summary of the code charts and the locations of blocks of characters associated with different scripts or sets of symbols.

Next, the chapter discusses the issue of writing direction and introduces several special types of characters important for understanding the Unicode Standard. In particular, the use of combining characters, the byte order mark, and other special characters is explored in some detail.

The section on equivalent sequences and normalization describes the issue of multiple equivalent representations of Unicode text and explains how text can be transformed to use a unique and preferred representation for each character sequence.

Finally, there is an informal statement of the conformance requirements for the Unicode Standard. This informal statement, with a number of easy-to-understand examples, gives a general sense of what conformance to the Unicode Standard means. The rigorous, formal definition of conformance is given in the subsequent Chapter 3, Conformance.

#2.1 Architectural Context

A character code standard such as the Unicode Standard enables the implementation of useful processes operating on textual data. The interesting end products are not the character codes but rather the text processes, because these directly serve the needs of a system’s users. Character codes are like nuts and bolts—minor, but essential and ubiquitous components used in many different ways in the construction of computer software systems. No single design of a character set can be optimal for all uses, so the architecture of the Unicode Standard strikes a balance among several competing requirements.

#2.1.1 Basic Text Processes

Most computer systems provide low-level functionality for a small number of basic text processes from which more sophisticated text-processing capabilities are built. The following text processes are supported by most computer systems to some degree:

• Rendering characters visible (including ligatures, contextual forms, and so on)
• Breaking lines while rendering (including hyphenation)
• Modifying appearance, such as point size, kerning, underlining, slant, and weight (light, demi, bold, and so on)
• Determining units such as “word” and “sentence”
• Interacting with users in processes such as selecting and highlighting text
• Accepting keyboard input and editing stored text through insertion and deletion
• Comparing text in operations such as in searching or determining the sort order of two strings
• Analyzing text content in operations such as spell-checking, hyphenation, and parsing morphology (that is, determining word roots, stems, and affixes)
• Treating text as bulk data for operations such as compressing and decompressing, truncating, transmitting, and receiving

#2.1.2 Text Elements, Characters, and Text Processes

One of the more profound challenges in designing a character encoding stems from the fact that there is no universal set of fundamental units of text. Instead, the division of text into text elements necessarily varies by language and text process.

For example, in the pre-1996 German orthography, the letter combination “ck” was a text element for the process of hyphenation (where it appeared as “k-k”), but not for the process of sorting. In Spanish, the combination “ll” may be a text element for the traditional process of sorting (where it is sorted between “l” and “m”), but not for the process of rendering. In English, the letters “A” and “a” are usually distinct text elements for the process of rendering, but generally not distinct for the process of searching text. The text elements in a given language depend upon the specific text process; a text element for spell-checking may have different boundaries from a text element for sorting purposes. For example, in the phrase “the quick brown fox,” the sequence “fox” is a text element for the purpose of spell-checking.

In contrast, a character encoding standard provides a single set of fundamental units of encoding, to which it uniquely assigns numerical code points. These units, called assigned characters, are the smallest interpretable units of stored text. Text elements are then represented by a sequence of one or more characters.

Figure 2-1 illustrates the relationship between several different types of text elements and the characters used to represent those text elements.

The design of the character encoding must provide precisely the set of characters that allows programmers to design applications capable of implementing a variety of text processes in the desired languages. Therefore, the text elements encountered in most text processes are represented as sequences of character codes. See Unicode Standard Annex #29, “Unicode Text Segmentation,” for detailed information on how to segment character strings into common types of text elements. Certain text elements correspond to what users perceive as single characters. These are called grapheme clusters.

#2.1.3 Text Processes and Encoding

In the case of English text using an encoding scheme such as ASCII, the relationships between the encoding and the basic text processes built on it are seemingly straightforward: characters are generally rendered visible one by one in distinct rectangles from left to right in linear order. Thus one character code inside the computer corresponds to one logical character in a process such as simple English rendering.

When designing an international and multilingual text encoding such as the Unicode Standard, the relationship between the encoding and implementation of basic text processes must be considered explicitly, for several reasons:

• Many assumptions about character rendering that hold true for the English alphabet fail for other writing systems. Characters in these other writing systems are not necessarily rendered visible one by one in rectangles from left to right. In many cases, character positioning is quite complex and does not proceed in a linear fashion. See Section 9.2, Arabic, and Section 12.1, Devanagari, for detailed examples of this situation.
• It is not always obvious that one set of text characters is an optimal encoding for a given language. For example, two approaches exist for the encoding of accented characters commonly used in French or Swedish: ISO/IEC 8859 defines letters such as “ä” and “ö” as individual characters, whereas ISO 5426 represents them by composition with diacritics instead. In the Swedish language, both are considered distinct letters of the alphabet, following the letter “z”. In French, the diaeresis on a vowel merely marks it as being pronounced in isolation. In practice, both approaches can be used to implement either language.
• No encoding can support all basic text processes equally well. As a result, some trade-offs are necessary. For example, following common practice, Unicode defines separate codes for uppercase and lowercase letters. This choice causes some text processes, such as rendering, to be carried out more easily, but other processes, such as comparison, to become more difficult. A different encoding design for English, such as case-shift control codes, would have the opposite effect. In designing a new encoding scheme for complex scripts, such trade-offs must be evaluated and decisions made explicitly.

For these reasons, design of the Unicode Standard is not specific to the design of particular basic text-processing algorithms. Instead, it provides an encoding that can be used with a wide variety of algorithms. In particular, sorting and string comparison algorithms cannot assume that the assignment of Unicode character code numbers provides an alphabetical ordering for lexicographic string comparison. Culturally expected sorting orders require arbitrarily complex sorting algorithms. The expected sort sequence for the same characters differs across languages; thus, in general, no single acceptable lexicographic ordering exists. See Unicode Technical Standard #10, “Unicode Collation Algorithm,” for the standard default mechanism for comparing Unicode strings.

Text processes supporting many languages are often more complex than they are for English. The character encoding design of the Unicode Standard strives to minimize this additional complexity, enabling modern computer systems to interchange, render, and manipulate text in a user’s own script and language—and possibly in other languages as well.

#Character Identity

Whenever Unicode makes statements about the default layout behavior of characters, it is done to ensure that users and implementers face no ambiguities as to which characters or character sequences to use for a given purpose. For bidirectional writing systems, this includes the specification of the sequence in which characters are to be encoded so as to correspond to a specific reading order when displayed. See Section 2.10, Writing Direction.

The actual layout in an implementation may differ in detail. A mathematical layout system, for example, will have many additional, domain-specific rules for layout, but a well-designed system leaves no ambiguities as to which character codes are to be used for a given aspect of the mathematical expression being encoded.

The purpose of defining Unicode default layout behavior is not to enforce a single and specific aesthetic layout for each script, but rather to encourage uniformity in encoding. In that way implementers of layout systems can rely on the fact that users would have chosen a particular character sequence for a given purpose, and users can rely on the fact that implementers will create a layout for a particular character sequence that matches the intent of the user to within the capabilities or technical limitations of the implementation.

In other words, two users who are familiar with the standard and who are presented with the same text ideally will choose the same sequence of character codes to encode the text. In actual practice there are many limitations, so this goal cannot always be realized.

#2.2 Unicode Design Principles

The design of the Unicode Standard reflects the 10 fundamental principles stated in Table 2-1. Not all of these principles can be satisfied simultaneously. The design strikes a balance between maintaining consistency for the sake of simplicity and efficiency and maintaining compatibility for interchange with existing standards.

Principle	Statement
Universality	The Unicode Standard provides a single, universal repertoire.
Efficiency	Unicode text is simple to parse and process.
Characters, not glyphs	The Unicode Standard encodes characters, not glyphs.
Semantics	Characters have well-defined semantics.
Plain text	Unicode characters represent plain text.
Logical order	The default for memory representation is logical order.
Unification	The Unicode Standard unifies duplicate characters within scripts across languages.
Dynamic composition	Accented forms can be dynamically composed.
Stability	Characters, once assigned, cannot be reassigned and key properties are immutable.
Convertibility	Accurate convertibility is guaranteed between the Unicode Standard and other widely accepted standards.

#2.2.1 Universality

The Unicode Standard encodes a single, very large set of characters, encompassing all the characters needed for worldwide use. This single repertoire is intended to be universal in coverage, containing all the characters for textual representation in all modern writing systems, in most historic writing systems, and for symbols used in plain text.

The Unicode Standard is designed to meet the needs of diverse user communities within each language, serving business, educational, liturgical and scientific users, and covering the needs of both modern and historical texts.

Despite its aim of universality, the Unicode Standard considers the following to be outside its scope: writing systems for which insufficient information is available to enable reliable encoding of characters, writing systems that have not become standardized through use, and writing systems that are nontextual in nature.

Because the universal repertoire is known and well defined in the standard, it is possible to specify a rich set of character semantics. By relying on those character semantics, implementations can provide detailed support for complex operations on text in a portable way. See “Semantics” later in this section.

#2.2.2 Efficiency

The Unicode Standard is designed to make efficient implementation possible. There are no escape characters or shift states in the Unicode character encoding model. Each character code has the same status as any other character code; all codes are equally accessible.

The standard Unicode encoding forms (UTF-8, UTF-16, and UTF-32) are self-synchronizing and non-overlapping. This makes randomly accessing and searching inside streams of characters efficient.

By convention, characters of a script are grouped together as far as is practical. Not only is this practice convenient for looking up characters in the code charts, but it makes implementations more compact and compression methods more efficient. The common punctuation characters are shared.

Format characters are given specific and unambiguous functions in the Unicode Standard. This design simplifies the support of subsets. To keep implementations simple and efficient, stateful controls and format characters are avoided wherever possible.

#2.2.3 Characters, Not Glyphs

The Unicode Standard draws a distinction between characters and glyphs. Characters are the abstract representations of the smallest components of written language that have semantic value. They represent primarily, but not exclusively, the letters, punctuation, and other signs that constitute natural language text and technical notation. The letters used in natural language text are grouped into scripts—sets of letters that are used together in writing languages. Letters in different scripts, even when they correspond either semantically or graphically, are represented in Unicode by distinct characters. This is true even in those instances where they correspond in semantics, pronunciation, or appearance.

Characters are represented by code points that reside only in a memory representation, as strings in memory, on disk, or in data transmission. The Unicode Standard deals only with character codes.

Glyphs represent the shapes that characters can have when they are rendered or displayed. In contrast to characters, glyphs appear on the screen or paper as particular representations of one or more characters. A repertoire of glyphs makes up a font. Glyph shape and methods of identifying and selecting glyphs are the responsibility of individual font vendors and of appropriate standards and are not part of the Unicode Standard.

Various relationships may exist between character and glyph: a single glyph may correspond to a single character or to a number of characters, or multiple glyphs may result from a single character. The distinction between characters and glyphs is illustrated in Figure 2-2.

Even the letter “a” has a wide variety of glyphs that can represent it. A lowercase Cyrillic “п” also has a variety of glyphs; the second glyph for U+043F CYRILLIC SMALL LETTER PE shown in Figure 2-2 is customary for italic in Russia, while the third is customary for italic in Serbia. Arabic letters are displayed with different glyphs, depending on their position in a word; the glyphs in Figure 2-2 show independent, final, initial, and medial forms. Sequences such as “fi” may be displayed with two independent glyphs or with a ligature glyph.

What the user thinks of as a single character—which may or may not be represented by a single glyph—may be represented in the Unicode Standard as multiple code points. See Table 2-2 for additional examples.

For certain scripts, such as Arabic and the various Indic scripts, the number of glyphs needed to display a given script may be significantly larger than the number of characters encoding the basic units of that script. The number of glyphs may also depend on the orthographic style supported by the font. For example, an Arabic font intended to support the Nastaliq style of Arabic script may possess many thousands of glyphs. However, the character encoding employs the same few dozen letters regardless of the font style used to depict the character data in context.

A font and its associated rendering process define an arbitrary mapping from Unicode characters to glyphs. Some of the glyphs in a font may be independent forms for individual characters; others may be rendering forms that do not directly correspond to any single character.

Text rendering requires that characters in memory be mapped to glyphs. The final appearance of rendered text may depend on context (neighboring characters in the memory representation), variations in typographic design of the fonts used, and formatting information (point size, superscript, subscript, and so on). The results on screen or paper can differ considerably from the prototypical shape of a letter or character, as shown in Figure 2-3.

For the Latin script, this relationship between character code sequence and glyph is relatively simple and well known; for several other scripts, it is documented in this standard. However, in all cases, fine typography requires a more elaborate set of rules than given here. The Unicode Standard documents the default relationship between character sequences and glyphic appearance for the purpose of ensuring that the same text content can be stored with the same, and therefore interchangeable, sequence of character codes.

#2.2.4 Semantics

Characters have well-defined semantics. These semantics are defined by explicitly assigned character properties, rather than implied through the character name or the position of a character in the code tables (see Section 3.5, Properties). The Unicode Character Database provides machine-readable character property tables for use in implementations of parsing, sorting, and other algorithms requiring semantic knowledge about the code points. These properties are supplemented by the description of script and character behavior in this standard. See also Unicode Technical Report #23, “The Unicode Character Property Model.”

The Unicode Standard identifies more than 100 different character properties, including numeric, casing, combination, and directionality properties (see Chapter 4, Character Properties). Additional properties may be defined as needed from time to time. Where characters are used in different ways in different languages, the relevant properties are normally defined outside the Unicode Standard. For example, Unicode Technical Standard #10, “Unicode Collation Algorithm,” defines a set of default collation weights that can be used with a standard algorithm. Tailorings for each language are provided in the Unicode Common Locale Data Repository (CLDR); see https://cldr.unicode.org.

The Unicode Standard, by supplying a universal repertoire associated with well-defined character semantics, does not require the code set independent model of internationalization and text handling. That model abstracts away string handling as manipulation of byte streams of unknown semantics to protect implementations from the details of hundreds of different character encodings and selectively late-binds locale-specific character properties to characters. Of course, it is always possible for code set independent implementations to retain their model and to treat Unicode characters as just another character set in that context. It is not at all unusual for Unix implementations to simply add UTF-8 as another character set, parallel to all the other character sets they support. By contrast, the Unicode approach—because it is associated with a universal repertoire—assumes that characters and their properties are inherently and inextricably associated. If an internationalized application can be structured to work directly in terms of Unicode characters, all levels of the implementation can reliably and efficiently access character storage and be assured of the universal applicability of character property semantics.

#2.2.5 Plain Text

Plain text is a pure sequence of character codes; plain Unicode-encoded text is therefore a sequence of Unicode character codes. In contrast, styled text, also known as rich text, is any text representation consisting of plain text plus added information such as a language identifier, font size, color, hypertext links, and so on. For example, the text of this specification, a multi-font text as formatted by a book editing system, is rich text.

The simplicity of plain text gives it a natural role as a major structural element of rich text. SGML, RTF, HTML, XML, and T_EX are examples of rich text fully represented as plain text streams, interspersing plain text data with sequences of characters that represent the additional data structures. They use special conventions embedded within the plain text file, such as “<p>”, to distinguish the markup or tags from the “real” content. Many popular word processing packages rely on a buffer of plain text to represent the content and implement links to a parallel store of formatting data.

The relative functional roles of both plain text and rich text are well established:

• Plain text is the underlying content stream to which formatting can be applied.
• Rich text carries complex formatting information as well as text context.
• Plain text is public, standardized, and universally readable.
• Rich text representation may be implementation-specific or proprietary.

Although some rich text formats have been standardized or made public, the majority of rich text designs are vehicles for particular implementations and are not necessarily readable by other implementations. Given that rich text equals plain text plus added information, the extra information in rich text can always be stripped away to reveal the “pure” text underneath. This operation is often employed, for example, in word processing systems that use both their own private rich text format and plain text file format as a universal, if limited, means of exchange. Thus, by default, plain text represents the basic, interchangeable content of text.

Plain text represents character content only, not its appearance. It can be displayed in a variety of ways and requires a rendering process to make it visible with a particular appearance. If the same plain text sequence is given to disparate rendering processes, there is no expectation that rendered text in each instance should have the same appearance. Instead, the disparate rendering processes are simply required to make the text legible according to the intended reading. This legibility criterion constrains the range of possible appearances. The relationship between appearance and content of plain text may be summarized as follows:

Plain text must contain enough information to permit the text to be rendered legibly, and nothing more.

The Unicode Standard encodes plain text. The distinction between plain text and other forms of data in the same data stream is the function of a higher-level protocol and is not specified by the Unicode Standard itself.

#2.2.6 Logical Order

The order in which Unicode text is stored in the memory representation is called logical order. This order roughly corresponds to the order in which text is typed in via the keyboard; it also roughly corresponds to phonetic order. For decimal numbers, the logical order consistently corresponds to the most significant digit first, which is the order expected by number-parsing software.

When displayed, this logical order often corresponds to a simple linear progression of characters in one direction, such as from left to right, right to left, or top to bottom. In other circumstances, text is displayed or printed in an order that differs from a single linear progression. Some of the clearest examples are situations where a right-to-left script (such as Arabic or Hebrew) is mixed with a left-to-right script (such as Latin or Greek). For example, when the text in Figure 2-4 is ordered for display the glyph that represents the first character of the English text appears at the left. The logical start character of the Hebrew text, however, is represented by the Hebrew glyph closest to the right margin. The succeeding Hebrew glyphs are laid out to the left.

In logical order, numbers are encoded with most significant digit first, but are displayed in different writing directions. As shown in Figure 2-5 these writing directions do not always correspond to the writing direction of the surrounding text. The first example shows N’Ko, a right-to-left script with digits that also render right to left. Examples 2 and 3 show Hebrew and Arabic, in which the numbers are rendered left to right, resulting in bidirectional layout. In left-to-right scripts, such as Latin and Hiragana and Katakana (for Japanese), numbers follow the predominant left-to-right direction of the script, as shown in Examples 4 and 5. When Japanese is laid out vertically, numbers are either laid out vertically or may be rotated clockwise ninety degrees to follow the layout direction of the lines, as shown in Example 6.

The Unicode Standard precisely defines the conversion of Unicode text from logical order to the order of readable (displayed) text so as to ensure consistent legibility. Properties of directionality inherent in characters generally determine the correct display order of text. The Unicode Bidirectional Algorithm specifies how these properties are used to resolve directional interactions when characters of right-to-left and left-to-right directionality are mixed. (See Unicode Standard Annex #9, “Unicode Bidirectional Algorithm.”) However, when characters of different directionality are mixed, inherent directionality alone is occasionally insufficient to render plain text legibly. The Unicode Standard therefore includes characters to explicitly specify changes in direction when necessary. The Bidirectional Algorithm uses these directional layout control characters together with the inherent directional properties of characters to exert exact control over the display ordering for legible interchange. By requiring the use of this algorithm, the Unicode Standard ensures that plain text used for simple items like file names or labels can always be correctly ordered for display.

Besides mixing runs of differing overall text direction, there are many other cases where the logical order does not correspond to a linear progression of characters. Combining characters (such as accents) are stored following the base character to which they apply, but are positioned relative to that base character and thus do not follow a simple linear progression in the final rendered text. For example, the Latin letter “x̣” is stored as “x” followed by combining “̣”; the accent appears below, not to the right of the base. This position with respect to the base holds even where the overall text progression is from top to bottom—for example, with “x̣” appearing upright within a vertical Japanese line. Characters may also combine into ligatures or conjuncts or otherwise change positions of their components radically, as shown in Figure 2-3 and Figure 2-19.

There is one particular exception to the usual practice of logical order paralleling phonetic order. With the Thai, Lao, Tai Viet, and New Tai Lue scripts, users traditionally type in visual order rather than phonetic order, resulting in some vowel letters being stored ahead of consonants, even though they are pronounced after them.

#2.2.7 Unification

The Unicode Standard avoids duplicate encoding of characters by unifying them within scripts across language. Common letters are given one code each, regardless of language, as are common Chinese/Japanese/Korean (CJK) ideographs. (See Section 18.1, Han.)

Punctuation marks, symbols, and diacritics are handled in a similar manner as letters. If they can be clearly identified with a particular script, they are encoded once for that script and are unified across any languages that may use that script. See, for example, U+1362 ETHIOPIC FULL STOP, U+060F ARABIC SIGN MISRA, and U+0592 HEBREW ACCENT SEGOL. However, some punctuation or diacritical marks may be shared in common across a number of scripts—the obvious example being Western-style punctuation characters, which are often recently added to the writing systems of scripts other than Latin. In such cases, characters are encoded only once and are intended for use with multiple scripts. Common symbols are also encoded only once and are not associated with any script in particular.

It is quite normal for many characters to have different usages, such as comma “ , ” for either thousands-separator (English) or decimal-separator (French). The Unicode Standard avoids duplication of characters due to specific usage in different languages; rather, it duplicates characters only to support compatibility with base standards. Avoidance of duplicate encoding of characters is important to avoid visual ambiguity.

There are a few notable instances in the standard where visual ambiguity between different characters is tolerated, however. For example, in most fonts there is little or no distinction visible between Latin “o”, Cyrillic “o”, and Greek “o” (omicron). These are not unified because they are characters from three different scripts, and many legacy character encodings distinguish between them. As another example, there are three characters whose glyph is the same uppercase barred D shape, but they correspond to three distinct lowercase forms. Unifying these uppercase characters would have resulted in unnecessary complications for case mapping.

The Unicode Standard does not attempt to encode features such as language, font, size, positioning, glyphs, and so forth. For example, it does not preserve language as a part of character encoding: just as French i grec, German ypsilon, and English wye are all represented by the same character code, U+0059 “Y”, so too are Chinese zi, Japanese ji, and Korean ja all represented as the same character code, U+5B57 字.

In determining whether to unify variant CJK ideograph forms across standards, the Unicode Standard follows the principles described in Section 18.1, Han. Where these principles determine that two forms constitute a trivial difference, the Unicode Standard assigns a single code. Just as for the Latin and other scripts, typeface distinctions or local preferences in glyph shapes alone are not sufficient grounds for disunification of a character. Figure 2-6 illustrates the well-known example of the CJK ideograph for “bone,” which shows significant shape differences from typeface to typeface, with some forms preferred in China and some in Japan. All of these forms are considered to be the same character, encoded at U+9AA8 in the Unicode Standard.

Many characters in the Unicode Standard could have been unified with existing visually similar Unicode characters or could have been omitted in favor of some other Unicode mechanism for maintaining the kinds of text distinctions for which they were intended. However, considerations of interoperability with other standards and systems often require that such compatibility characters be included in the Unicode Standard. See Section 2.3, Compatibility Characters. In particular, whenever font style, size, positioning or precise glyph shape carry a specific meaning and are used in distinction to the ordinary character—for example, in phonetic or mathematical notation—the characters are not unified.

#2.2.8 Dynamic Composition

The Unicode Standard allows for the dynamic composition of accented forms and Hangul syllables. Combining characters used to create composite forms are productive. Because the process of character composition is open-ended, new forms with modifying marks may be created from a combination of base characters followed by combining characters. For example, the diaeresis “¨” may be combined with all vowels and a number of consonants in languages using the Latin script and several other scripts, as shown in Figure 2-7.

#Equivalent Sequences

Some text elements can be encoded either as static precomposed forms or by dynamic composition. Common precomposed forms such as U+00DC “Ü” LATIN CAPITAL LETTER U WITH DIAERESIS are included for compatibility with current standards. For static precomposed forms, the standard provides a mapping to an equivalent dynamically composed sequence of characters. (See also Section 3.7, Decomposition.) Thus different sequences of Unicode characters are considered equivalent. A precomposed character may be represented as an equivalent composed character sequence (see Section 2.12, Equivalent Sequences).

#2.2.9 Stability

Certain aspects of the Unicode Standard must be absolutely stable between versions, so that implementers and users can be guaranteed that text data, once encoded, retains the same meaning. Most importantly, this means that once Unicode characters are assigned, their code point assignments cannot be changed, nor can characters be removed.

Characters are retained in the standard, so that previously conforming data stay conformant in future versions of the standard. Sometimes characters are deprecated—that is, their use in new documents is strongly discouraged. While implementations should continue to recognize such characters when they are encountered, spell-checkers or editors could warn users of their presence and suggest replacements. For more about deprecated characters, see D13 in Section 3.4, Characters and Encoding.

Unicode character names are also never changed, so that they can be used as identifiers that are valid across versions. See Section 4.8, Name.

Similar stability guarantees exist for certain important properties. For example, the decompositions are kept stable, so that it is possible to normalize a Unicode text once and have it remain normalized in all future versions.

The most current versions of the character encoding stability policies for the Unicode Standard are maintained online at:

https://www.unicode.org/policies/stability_policy.html

#2.2.10 Convertibility

Character identity is preserved for interchange with a number of different base standards, including national, international, and vendor standards. Where variant forms (or even the same form) are given separate codes within one base standard, they are also kept separate within the Unicode Standard. This choice guarantees the existence of a mapping between the Unicode Standard and base standards.

Accurate convertibility is guaranteed between the Unicode Standard and other standards in wide usage as of May 1993. Characters have also been added to allow convertibility to several important East Asian character sets created after that date—for example, GB 18030. In general, a single code point in another standard will correspond to a single code point in the Unicode Standard. Sometimes, however, a single code point in another standard corresponds to a sequence of code points in the Unicode Standard, or vice versa. Conversion between Unicode text and text in other character codes must, in general, be done by explicit table-mapping processes. (See also Section 5.1, Data Structures for Character Conversion.)

#2.3 Compatibility Characters

Conceptually, compatibility characters are characters that would not have been encoded in the Unicode Standard except for compatibility and round-trip convertibility with other standards. Such standards include international, national, and vendor character encoding standards. For the most part, these are widely used standards that pre-dated Unicode, but because continued interoperability with new standards and data sources is one of the primary design goals of the Unicode Standard, additional compatibility characters are added as the situation warrants.

Compatibility characters can be contrasted with ordinary (or non-compatibility) characters in the standard—ones that are generally consistent with the Unicode text model and which would have been accepted for encoding to represent various scripts and sets of symbols, regardless of whether those characters also existed in other character encoding standards.

For example, in the Unicode model of Arabic text the logical representation of text uses basic Arabic letters. Rather than being directly represented in the encoded characters, the cursive presentation of Arabic text for display is determined in context by a rendering system. (See Section 9.2, Arabic.) However, some earlier character encodings for Arabic were intended for use with rendering systems that required separate characters for initial, medial, final, and isolated presentation forms of Arabic letters. To allow one-to-one mapping to these character sets, the Unicode Standard includes Arabic presentation forms as compatibility characters.

The purpose for the inclusion of compatibility characters like these is not to implement or emulate alternative text models, nor to encourage the use of plain text distinctions in characters which would otherwise be better represented by higher-level protocols or other mechanisms. Rather, the main function of compatibility characters is to simplify interoperability of Unicode-based systems with other data sources, and to ensure convertibility of data.

Interoperability does not require that all external characters can be mapped to single Unicode characters; encoding a compatibility character is not necessary when a character in another standard can be represented as a sequence of existing Unicode characters. For example the Shift-JIS encoding 0x839E for JIS X 0213 katakana letter ainu to can simply be mapped to the Unicode character sequence <U+30C8, U+309A>. However, in cases where no appropriate mapping is available, the requirement for interoperability and convertibility may be met by encoding a compatibility character for one-to-one mapping to another standard.

#Usage

The fact that a particular character is considered a compatibility character does not mean that that character is deprecated in the standard. The use of most compatibility characters in general text interchange is unproblematic. Some, however, such as the Arabic positional forms or other compatibility characters which assume information about particular layout conventions, such as presentation forms for vertical text, can lead to problems when used in general interchange. Caution is advised for their use. See also the discussion of compatibility characters in the W3C specification, “Unicode in XML and Other Markup Languages.”

#Allocation

The Compatibility and Specials Area contains a large number of compatibility characters, but the Unicode Standard also contains many compatibility characters that do not appear in that area. These include examples such as U+2163 “IV” ROMAN NUMERAL FOUR, U+2007 FIGURE SPACE, U+00B2 “²” SUPERSCRIPT TWO, U+2502 BOX DRAWINGS LIGHT VERTICAL, and U+32D0 CIRCLED KATAKANA A.

There is no formal listing of all compatibility characters in the Unicode Standard. This follows from the nature of the definition of compatibility characters. It is a judgement call as to whether any particular character would have been accepted for encoding if it had not been required for interoperability with a particular standard. Different participants in character encoding often disagree about the appropriateness of encoding particular characters, and sometimes there are multiple justifications for encoding a given character.

#2.3.1 Compatibility Variants

Compatibility variants are a subset of compatibility characters, and have the further characteristic that they represent variants of existing, ordinary, Unicode characters.

For example, compatibility variants might represent various presentation or styled forms of basic letters: superscript or subscript forms, variant glyph shapes, or vertical presentation forms. They also include halfwidth or fullwidth characters from East Asian character encoding standards, Arabic contextual form glyphs from preexisting Arabic code pages, Arabic ligatures and ligatures from other scripts, and so on. Compatibility variants also include CJK compatibility ideographs, many of which are minor glyph variants of an encoded unified CJK ideograph.

In contrast to compatibility variants there are the numerous compatibility characters, such as U+2502 BOX DRAWINGS LIGHT VERTICAL, U+263A WHITE SMILING FACE, or U+2701 UPPER BLADE SCISSORS, which are not variants of ordinary Unicode characters. However, it is not always possible to determine unequivocally whether a compatibility character is a variant or not.

#2.3.2 Compatibility Decomposable Characters

The term compatibility is further applied to Unicode characters in a different, strictly defined sense. The concept of a compatibility decomposable character is formally defined as any Unicode character whose compatibility decomposition is not identical to its canonical decomposition. (See Definition D66 in Section 3.7, Decomposition, and the discussion in Section 2.2, Unicode Design Principles.)

The list of compatibility decomposable characters is precisely defined by property values in the Unicode Character Database, and by the rules of Unicode Normalization. (See Section 3.11, Normalization Forms.) Because of their use in Unicode Normalization, compatibility decompositions are stable and cannot be changed once a character has been encoded; the list of compatibility decomposable characters for any version of the Unicode Standard is thus also stable.

Compatibility decomposable characters have also been referred to in earlier versions of the Unicode Standard as compatibility composite characters or compatibility composites for short, but the full term, compatibility decomposable character is preferred.

#Compatibility Character Vs. Compatibility Decomposable Character

In informal discussions of the Unicode Standard, compatibility decomposable characters have also often been referred to simply as “compatibility characters.” This is understandable, in part because the two sets of characters largely overlap, but the concepts are actually distinct. There are compatibility characters which are not compatibility decomposable characters, and there are compatibility decomposable characters which are not compatibility characters.

For example, the deprecated alternate format characters such as U+206C INHIBIT ARABIC FORM SHAPING are considered compatibility characters, but they have no decomposition mapping, and thus by definition cannot be compatibility decomposable characters. Likewise for such other compatibility characters as U+2502 BOX DRAWINGS LIGHT VERTICAL or U+263A WHITE SMILING FACE.

There are also instances of compatibility variants which clearly are variants of other Unicode characters, but which have no decomposition mapping. For example, U+2EAF CJK RADICAL SILK is a compatibility variant of U+2F77 KANGXI RADICAL SILK, as well as being a compatibility variant of U+7CF9 CJK UNIFIED IDEOGRAPH-7CF9, but has no compatibility decomposition. The numerous compatibility variants like this in the CJK Radicals Supplement block were encoded for compatibility with encodings that distinguished and separately encoded various forms of CJK radicals as symbols.

A different case is illustrated by the CJK compatibility ideographs, such as U+FA0C CJK COMPATIBILITY IDEOGRAPH-FA0C. Those compatibility characters have a decomposition mapping, but for historical reasons it is always a canonical decomposition, so they are canonical decomposable characters, but not compatibility decomposable characters.

By way of contrast, some compatibility decomposable characters, such as modifier letters used in phonetic orthographies, for example, U+02B0 MODIFIER LETTER SMALL H, are not considered to be compatibility characters. They would have been accepted for encoding in the standard on their own merits, regardless of their need for mapping to IPA. A large number of compatibility decomposable characters like this are actually distinct symbols used in specialized notations, whether phonetic or mathematical. In such cases, their compatibility mappings express their historical derivation from styled forms of standard letters.

Other compatibility decomposable characters are widely used characters serving essential functions. U+00A0 NO-BREAK SPACE is one example. In these and similar cases, such as fixed-width space characters, the compatibility decompositions define possible fallback representations.

The Unicode Character Database supplies identification and mapping information only for compatibility decomposable characters, while compatibility variants are not formally identified or documented. Because the two sets substantially overlap, many specifications are written in terms of compatibility decomposable characters first; if necessary, such specifications may be extended to handle other, non-decomposable compatibility variants as required. (See also the discussion in Section 5.19, Mapping Compatibility Variants.)

#2.4 Code Points and Characters

On a computer, abstract characters are encoded internally as numbers. To create a complete character encoding, it is necessary to define the list of all characters to be encoded and to establish systematic rules for how the numbers represent the characters.

The range of integers used to code the abstract characters is called the codespace. A particular integer in this set is called a code point. When an abstract character is mapped or assigned to a particular code point in the codespace, it is then referred to as an encoded character.

In the Unicode Standard, the codespace consists of the integers from 0 to 10FFFF₁₆, comprising 1,114,112 code points available for assigning the repertoire of abstract characters.

There are constraints on how the codespace is organized, and particular areas of the codespace have been set aside for encoding of certain kinds of abstract characters or for other uses in the standard. For more on the allocation of the Unicode codespace, see Section 2.8, Unicode Allocation.

Figure 2-8 illustrates the relationship between abstract characters and code points, which together constitute encoded characters. Note that some abstract characters may be associated with multiple, separately encoded characters (that is, be encoded “twice”). In other instances, an abstract character may be represented by a sequence of two (or more) other encoded characters. The solid arrows connect encoded characters with the abstract characters that they represent and encode.

When referring to code points in the Unicode Standard, the usual practice is to refer to them by their numeric value expressed in hexadecimal, with a “U+” prefix. (See Appendix A, Notational Conventions.) Encoded characters can also be referred to by their code points only. To prevent ambiguity, the official Unicode name of the character is often added; this clearly identifies the abstract character that is encoded. For example:

U+0061 LATIN SMALL LETTER A

U+10330 GOTHIC LETTER AHSA

U+201DF CJK UNIFIED IDEOGRAPH-201DF

Such citations refer only to the encoded character per se, associating the code point (as an integral value) with the abstract character that is encoded.

#2.4.1 Types of Code Points

There are many ways to categorize code points. Table 2-3 illustrates some of the categorizations and basic terminology used in the Unicode Standard. The seven basic types of code points are formally defined in Section 3.4, Characters and Encoding. (See Definition D10a, Code Point Type.)

Basic Type	Brief Description	General Category	Character Status	Code Point Status
Graphic	Letter, mark, number, punctuation, symbol, and spaces	L, M, N, P, S, Zs	Assigned to abstract character	Designated (assigned) code point
Format	Invisible but affects neighboring characters; includes line/paragraph separators	Cf, Zl, Zp
Control	Usage defined by protocols or standards outside the Unicode Standard	Cc
Private-use	Usage defined by private agreement outside the Unicode Standard	Co
Surrogate	Permanently reserved for UTF-16; restricted interchange	Cs	Cannot be assigned to abstract character
Noncharacter	Permanently reserved for internal usage; restricted interchange	Cn	Not assigned to abstract character
Reserved	Reserved for future assignment; restricted interchange			Undesignated (unassigned) code point

Not all assigned code points represent abstract characters; only Graphic, Format, Control and Private-use do. Surrogates and Noncharacters are assigned code points but are not assigned to abstract characters. Reserved code points are assignable: any may be assigned in a future version of the standard. The General Category provides a finer breakdown of Graphic characters and also distinguishes between the other basic types (except between Noncharacter and Reserved). Other properties defined in the Unicode Character Database provide for different categorizations of Unicode code points.

#Control Codes

Sixty-five code points (U+0000..U+001F and U+007F..U+009F) are defined specifically as control codes, for compatibility with the C0 and C1 control codes of the ISO/IEC 2022 framework. A few of these control codes are given specific interpretations by the Unicode Standard. (See Section 23.1, Control Codes.)

#Noncharacters

Sixty-six code points are not used to encode characters. Noncharacters consist of U+FDD0..U+FDEF and any code point ending in the value FFFE₁₆ or FFFF₁₆—that is, U+FFFE, U+FFFF, U+1FFFE, U+1FFFF, ... U+10FFFE, U+10FFFF. (See Section 23.7, Noncharacters.)

#Private Use

Three ranges of code points have been set aside for private use. Characters in these areas will never be defined by the Unicode Standard. These code points can be freely used for characters of any purpose, but successful interchange requires an agreement between sender and receiver on their interpretation. (See Section 23.5, Private-Use Characters.)

#Surrogates

Some 2,048 code points have been allocated as surrogate code points, which are used in the UTF-16 encoding form. (See Section 23.6, Surrogates Area.)

#Restricted Interchange

Code points that are not assigned to abstract characters are subject to restrictions in interchange.

• Surrogate code points cannot be conformantly interchanged using Unicode encoding forms. They do not correspond to Unicode scalar values and thus do not have well-formed representations in any Unicode encoding form. (See Section 3.8, Surrogates.)
• Noncharacter code points are reserved for internal use, such as for sentinel values. They have well-formed representations in Unicode encoding forms and survive conversions between encoding forms. This allows sentinel values to be preserved internally across Unicode encoding forms, even though they are not designed to be used in open interchange.
• All implementations need to preserve reserved code points because they may originate in implementations that use a future version of the Unicode Standard. For example, suppose that one person is using a Unicode 12.0 system and a second person is using a Unicode 11.0 system. The first person sends the second person a document containing some code points newly assigned in Unicode 12.0; these code points were unassigned in Unicode 11.0. The second person may edit the document, not changing the reserved codes, and send it on. In that case the second person is interchanging what are, as far as the second person knows, reserved code points.

#Code Point Semantics

The semantics of most code points are established by this standard; the exceptions are Controls, Private-use, and Noncharacters. Control codes generally have semantics determined by other standards or protocols (such as ISO/IEC 6429), but there are a small number of control codes for which the Unicode Standard specifies particular semantics. See Table 23-1 in Section 23.1, Control Codes, for the exact list of those control codes. The semantics of private-use characters are outside the scope of the Unicode Standard; their use is determined by private agreement, as, for example, between vendors. Noncharacters have semantics in internal use only.

#2.5 Encoding Forms

Computers handle numbers not simply as abstract mathematical objects, but as combinations of fixed-size units like bytes and 32-bit words. A character encoding model must take this fact into account when determining how to associate numbers with the characters.

Actual implementations in computer systems represent integers in specific code units of particular size—usually 8-bit (= byte), 16-bit, or 32-bit. In the Unicode character encoding model, precisely defined encoding forms specify how each integer (code point) for a Unicode character is to be expressed as a sequence of one or more code units. The Unicode Standard provides three distinct encoding forms for Unicode characters, using 8-bit, 16-bit, and 32-bit units. These are named UTF-8, UTF-16, and UTF-32, respectively. The “UTF” is a carryover from earlier terminology meaning Unicode (or UCS) Transformation Format. Each of these three encoding forms is an equally legitimate mechanism for representing Unicode characters; each has advantages in different environments.

All three encoding forms can be used to represent the full range of encoded characters in the Unicode Standard; they are thus fully interoperable for implementations that may choose different encoding forms for various reasons. Each of the three Unicode encoding forms can be efficiently transformed into either of the other two without any loss of data.

#Non-overlap

Each of the Unicode encoding forms is designed with the principle of non-overlap in mind. Figure 2-9 presents an example of an encoding where overlap is permitted. In this encoding (Windows code page 932), characters are formed from either one or two code bytes. Whether a sequence is one or two bytes in length depends on the first byte, so that the values for lead bytes (of a two-byte sequence) and single bytes are disjoint. However, single-byte values and trail-byte values can overlap. That means that when someone searches for the character “D”, for example, he or she might find it either (mistakenly) as the trail byte of a two-byte sequence or as a single, independent byte. To find out which alternative is correct, a program must look backward through text.

The situation is made more complex by the fact that lead and trail bytes can also overlap, as shown in the second part of Figure 2-9. This means that the backward scan has to repeat until it hits the start of the text or hits a sequence that could not exist as a pair as shown in Figure 2-10. This is not only inefficient, but also extremely error-prone: corruption of one byte can cause entire lines of text to be corrupted.

The Unicode encoding forms avoid this problem, because none of the ranges of values for the lead, trail, or single code units in any of those encoding forms overlap.

Non-overlap makes all of the Unicode encoding forms well behaved for searching and comparison. When searching for a particular character, there will never be a mismatch against some code unit sequence that represents just part of another character. The fact that all Unicode encoding forms observe this principle of non-overlap distinguishes them from many legacy East Asian multibyte character encodings, for which overlap of code unit sequences may be a significant problem for implementations.

Another aspect of non-overlap in the Unicode encoding forms is that all Unicode characters have determinate boundaries when expressed in any of the encoding forms. That is, the edges of code unit sequences representing a character are easily determined by local examination of code units; there is never any need to scan back indefinitely in Unicode text to correctly determine a character boundary. This property of the encoding forms has sometimes been referred to as self-synchronization. This property has another very important implication: corruption of a single code unit corrupts only a single character; none of the surrounding characters are affected.

For example, when randomly accessing a string, a program can find the boundary of a character with limited backup. In UTF-16, if a pointer points to a leading surrogate, a single backup is required. In UTF-8, if a pointer points to a byte starting with 10xxxxxx (in binary), one to three backups are required to find the beginning of the character.

#Conformance

The Unicode Consortium fully endorses the use of any of the three Unicode encoding forms as a conformant way of implementing the Unicode Standard. It is important not to fall into the trap of trying to distinguish “UTF-8 versus Unicode,” for example. UTF-8, UTF-16, and UTF-32 are all equally valid and conformant ways of implementing the encoded characters of the Unicode Standard.

#Examples

Figure 2-11 shows the three Unicode encoding forms, including how they are related to Unicode code points.

In Figure 2-11, the UTF-32 line shows that each example character can be expressed with one 32-bit code unit. Those code units have the same values as the code point for the character. For UTF-16, most characters can be expressed with one 16-bit code unit, whose value is the same as the code point for the character, but characters with high code point values require a pair of 16-bit surrogate code units instead. In UTF-8, a character may be expressed with one, two, three, or four bytes, and the relationship between those byte values and the code point value is more complex.

UTF-8, UTF-16, and UTF-32 are further described in the subsections that follow. See each subsection for a general overview of how each encoding form is structured and the general benefits or drawbacks of each encoding form for particular purposes. For the detailed formal definition of the encoding forms and conformance requirements, see Section 3.9, Unicode Encoding Forms.

#2.5.1 UTF-32

UTF-32 is the simplest Unicode encoding form. Each Unicode code point is represented directly by a single 32-bit code unit. Because of this, UTF-32 has a one-to-one relationship between encoded character and code unit; it is a fixed-width character encoding form.

Implementations that generally use UTF-8 or UTF-16 sometimes temporarily convert strings to UTF-32 for easier processing.

In some use cases, multi-code point sequences are useful or necessary as units of processing; for example, grapheme clusters, or sequences of characters with non-zero combining classes. This may limit the usefulness of a per-code point fixed-width encoding.

String encoding forms like UTF-32 or UTF-8 are irrelevant for APIs that pass single character values: These typically take or return simple code point integers.

As for all of the Unicode encoding forms, UTF-32 is restricted to representation of code points in the ranges 0..D7FF₁₆ and E000₁₆..10FFFF₁₆—that is, Unicode scalar values. This guarantees interoperability with the UTF-16 and UTF-8 encoding forms.

#Fixed Width

The value of each UTF-32 code unit corresponds exactly to the Unicode code point value. This situation differs significantly from that for UTF-16 and especially UTF-8, where the code unit values often change unrecognizably from the code point value. For example, U+10000 is represented as <00010000> in UTF-32 and as <F0 90 80 80> in UTF-8. For UTF-32, it is trivial to determine a Unicode character from its UTF-32 code unit representation. In contrast, UTF-16 and UTF-8 representations often require doing a code unit conversion before the character can be identified in the Unicode code charts.

#Preferred Usage

UTF-32 may be a preferred encoding form where memory or disk storage space for characters is not a particular concern, but where fixed-width, single code unit access to characters is desired. For example, Python 3 strings are sequences of Unicode code points.

#2.5.2 UTF-16

In the UTF-16 encoding form, non-surrogate code points in the range U+0000..U+FFFF are represented as a single 16-bit code unit; code points in the supplementary planes, in the range U+10000..U+10FFFF, are represented as pairs of 16-bit code units. These pairs of special code units are known as surrogate pairs. The values of the code units used for surrogate pairs are completely disjunct from the code units used for the single code unit representations, thus maintaining non-overlap for all code point representations in UTF-16. For the formal definition of surrogates, see Section 3.8, Surrogates.

#Optimized for BMP

UTF-16 optimizes the representation of characters in the Basic Multilingual Plane (BMP)—that is, characters in the range U+0000..U+FFFF, excluding surrogate code points. For that range, which contains the vast majority of common-use characters for all modern scripts of the world, each character requires only one 16-bit code unit, thus requiring just half the memory or storage of the UTF-32 encoding form. For the BMP, UTF-16 can effectively be treated as if it were a fixed-width encoding form.

#Supplementary Characters and Surrogates

For supplementary characters, UTF-16 requires two 16-bit code units. The distinction between characters represented with one versus two 16-bit code units means that UTF-16 is a variable-width encoding form. That fact can create implementation difficulties if it is not carefully taken into account; UTF-16 is somewhat more complicated to handle than UTF-32.

#Preferred Usage

UTF-16 may be a preferred encoding form in many environments that need to balance efficient access to characters with economical use of storage. It is reasonably compact, and all the common, heavily used characters fit into a single 16-bit code unit.

#Origin

UTF-16 is the historical descendant of the earliest form of Unicode, which was originally designed to use a fixed-width, 16-bit encoding form exclusively. The surrogates were added to provide an encoding form for the supplementary characters at code points past U+FFFF. The design of the surrogates made them a simple and efficient extension mechanism that works well with older Unicode implementations and that avoids many of the problems of other variable-width character encodings. See Section 5.4, Handling Surrogate Pairs in UTF-16, for more information about surrogates and their processing.

#Binary Sorting

For the purpose of sorting text, if the text contains supplementary code points, binary order for data represented in the UTF-16 encoding form is not the same as code point order. This means that a slightly different comparison implementation is needed for code point order. For more information, see Section 5.17, Binary Order.

#2.5.3 UTF-8

To meet the requirements of byte-oriented, ASCII-based systems, a third encoding form is specified by the Unicode Standard: UTF-8. This variable-width encoding form preserves ASCII transparency by making use of 8-bit code units.

#Byte-Oriented

Much existing software and practice in information technology have long depended on character data being represented as a sequence of bytes. Furthermore, many of the protocols depend not only on ASCII values being invariant, but must make use of or avoid special byte values that may have associated control functions. The easiest way to adapt Unicode implementations to such a situation is to make use of an encoding form that is already defined in terms of 8-bit code units and that represents all Unicode characters while not disturbing or reusing any ASCII or C0 control code value. That is the function of UTF-8.

#Variable Width

UTF-8 is a variable-width encoding form, using 8-bit code units, in which the high bits of each code unit indicate the part of the code unit sequence to which each byte belongs. A range of 8-bit code unit values is reserved for the first, or leading, element of a UTF-8 code unit sequences, and a completely disjunct range of 8-bit code unit values is reserved for the subsequent, or trailing, elements of such sequences; this convention preserves non-overlap for UTF-8. Table 3-6 shows how the bits in a Unicode code point are distributed among the bytes in the UTF-8 encoding form. See Section 3.9, Unicode Encoding Forms, for the full, formal definition of UTF-8.

#ASCII Transparency

The UTF-8 encoding form maintains transparency for all of the ASCII code points (0x00..0x7F). That means Unicode code points U+0000..U+007F are converted to single bytes 0x00..0x7F in UTF-8 and are thus indistinguishable from ASCII itself. Furthermore, the values 0x00..0x7F do not appear in any byte for the representation of any other Unicode code point, so that there can be no ambiguity. Beyond the ASCII range of Unicode, many of the non-ideographic scripts are represented by two bytes per code point in UTF-8; all non-surrogate code points between U+0800 and U+FFFF are represented by three bytes; and supplementary code points above U+FFFF require four bytes.

#Data Size

UTF-8 is reasonably compact in terms of the number of bytes used. Compared with UTF-16, it is much smaller for ASCII syntax and Western languages, but significantly larger for Asian writing systems such as for Hindi, Thai, Chinese, Japanese, and Korean.

#Preferred Usage

UTF-8 is typically the preferred encoding form for HTML and similar protocols, particularly for the Internet. The ASCII transparency helps migration. UTF-8 also has the advantage that it is already inherently byte-serialized, as for most existing 8-bit character sets; strings of UTF-8 work easily with the C standard library, and many existing APIs that work for typical East Asian multibyte character sets adapt to UTF-8 as well with little or no change required.

#Self-synchronizing

In environments where 8-bit character processing is required for one reason or another, UTF-8 has the following attractive features as compared to other multibyte encodings:

• The first byte of a UTF-8 code unit sequence indicates the number of bytes to follow in a multibyte sequence. This allows for very efficient forward parsing.
• It is efficient to find the start of a character when beginning from an arbitrary location in a byte stream of UTF-8. Programs need to search at most four bytes backward, and usually much less. It is a simple task to recognize an initial byte, because initial bytes are constrained to a fixed range of values.
• As with the other encoding forms, there is no overlap of byte values.

#Binary Sorting

A binary sort of UTF-8 strings gives the same ordering as a binary sort of Unicode code points. This is obviously the same order as for a binary sort of UTF-32 strings.

#2.6 Encoding Schemes

The discussion of Unicode encoding forms in the previous section was concerned with the machine representation of Unicode code units. Each code unit is represented in a computer simply as a numeric data type; just as for other numeric types, the exact way the bits are laid out internally is irrelevant to most processing. However, interchange of textual data, particularly between computers of different architectural types, requires consideration of the exact ordering of the bits and bytes involved in numeric representation. Integral data, including character data, is serialized for open interchange into well-defined sequences of bytes. This process of byte serialization allows all applications to correctly interpret exchanged data and to accurately reconstruct numeric values (and thereby character values) from it. In the Unicode Standard, the specifications of the distinct types of byte serializations to be used with Unicode data are known as Unicode encoding schemes.

#Byte Order

Modern computer architectures differ in ordering in terms of whether the most significant byte or the least significant byte of a large numeric data type comes first in internal representation. These sequences are known as “big-endian” and “little-endian” orders, respectively. For the Unicode 16- and 32-bit encoding forms (UTF-16 and UTF-32), the specification of a byte serialization must take into account the big-endian or little-endian architecture of the system on which the data is represented, so that when the data is byte serialized for interchange it will be well defined.

A character encoding scheme consists of a specified character encoding form plus a specification of how the code units are serialized into bytes. The Unicode Standard also specifies the use of an initial byte order mark (BOM) to explicitly differentiate big-endian or little-endian data in some of the Unicode encoding schemes. (See the “Byte Order Mark” subsection in Section 23.8, Specials.)

When a higher-level protocol supplies mechanisms for handling the endianness of integral data types, it is not necessary to use Unicode encoding schemes or the byte order mark. In those cases Unicode text is simply a sequence of integral data types.

For UTF-8, the encoding scheme consists merely of the UTF-8 code units (= bytes) in sequence. Hence, there is no issue of big- versus little-endian byte order for data represented in UTF-8. However, for 16-bit and 32-bit encoding forms, byte serialization must break up the code units into two or four bytes, respectively, and the order of those bytes must be clearly defined. Because of this, and because of the rules for the use of the byte order mark, the three encoding forms of the Unicode Standard result in a total of seven Unicode encoding schemes, as shown in Table 2-4.

Encoding Scheme	Endian Order	BOM Allowed?
UTF-8	N/A	yes
UTF-16	Big-endian or little-endian	yes
UTF-16BE	Big-endian	no
UTF-16LE	Little-endian	no
UTF-32	Big-endian or little-endian	yes
UTF-32BE	Big-endian	no
UTF-32LE	Little-endian	no

The endian order entry for UTF-8 in Table 2-4 is marked N/A because UTF-8 code units are 8 bits in size, and the usual machine issues of endian order for larger code units do not apply. The serialized order of the bytes must not depart from the order defined by the UTF-8 encoding form. Use of a BOM is not required for UTF-8, but may be encountered in contexts where UTF-8 data is converted from other encoding forms that use a BOM or where the BOM is used as a UTF-8 signature. See the “Byte Order Mark” subsection in Section 23.8, Specials, for more information. The Unicode Standard does not recommend for or against use of a BOM in UTF-8 data.

#Encoding Scheme Versus Encoding Form

Note that some of the Unicode encoding schemes have the same labels as the three Unicode encoding forms. This could cause confusion, so it is important to keep the context clear when using these terms: character encoding forms refer to integral data units in memory or in APIs, and byte order is irrelevant; character encoding schemes refer to byte-serialized data, as for streaming I/O or in file storage, and byte order must be specified or determinable.

The Internet Assigned Numbers Authority (IANA) maintains a registry of charset names used on the Internet. Those charset names are very close in meaning to the Unicode character encoding model’s concept of character encoding schemes, and all of the Unicode character encoding schemes are, in fact, registered as charsets. While the two concepts are quite close and the names used are identical, some important differences may arise in terms of the requirements for each, particularly when it comes to handling of the byte order mark. Exercise due caution when equating the two.

#Examples

Figure 2-12 illustrates the Unicode character encoding schemes, showing how each is derived from one of the encoding forms by serialization of bytes.

In Figure 2-12, the code units used to express each example character have been serialized into sequences of bytes. This figure should be compared with Figure 2-11, which shows the same characters before serialization into sequences of bytes. The “BE” lines show serialization in big-endian order, whereas the “LE” lines show the bytes reversed into little-endian order. For UTF-8, the code unit is just an 8-bit byte, so that there is no distinction between big-endian and little-endian order. UTF-32 and UTF-16 encoding schemes using the byte order mark are not shown in Figure 2-12, to keep the basic picture regarding serialization of bytes clearer.

For the detailed formal definition of the Unicode encoding schemes and conformance requirements, see Section 3.10, Unicode Encoding Schemes. For further general discussion about character encoding forms and character encoding schemes, both for the Unicode Standard and as applied to other character encoding standards, see Unicode Technical Report #17, “Unicode Character Encoding Model.” For information about charsets and character conversion, see Unicode Technical Standard #22, “Character Mapping Markup Language (CharMapML).”

#2.7 Unicode Strings

A Unicode string data type is simply an ordered sequence of code units. Thus a Unicode 8-bit string is an ordered sequence of 8-bit code units, a Unicode 16-bit string is an ordered sequence of 16-bit code units, and a Unicode 32-bit string is an ordered sequence of 32-bit code units.

Depending on the programming environment, a Unicode string may or may not be required to be in the corresponding Unicode encoding form. For example, strings in Java, C#, or ECMAScript are Unicode 16-bit strings, but are not necessarily well-formed UTF-16 sequences. In normal processing, it can be far more efficient to allow such strings to contain code unit sequences that are not well-formed UTF-16—that is, isolated surrogates. Because strings are such a fundamental component of every program, checking for isolated surrogates in every operation that modifies strings can create significant overhead, especially because supplementary characters are extremely rare as a percentage of overall text in programs worldwide.

It is straightforward to design basic string manipulation libraries that handle isolated surrogates in a consistent and straightforward manner. They cannot ever be interpreted as abstract characters, but they can be internally handled the same way as noncharacters where they occur. Typically they occur only ephemerally, such as in dealing with keyboard events. While an ideal protocol would allow keyboard events to contain complete strings, many allow only a single UTF-16 code unit per event. As a sequence of events is transmitted to the application, a string that is being built up by the application in response to those events may contain isolated surrogates at any particular point in time.

Whenever such strings are specified to be in a particular Unicode encoding form—even one with the same code unit size—the string must not violate the requirements of that encoding form. For example, isolated surrogates in a Unicode 16-bit string are not allowed when that string is specified to be well-formed UTF-16. A number of techniques are available for dealing with an isolated surrogate, such as omitting it, converting it into U+FFFD REPLACEMENT CHARACTER to produce well-formed UTF-16, or simply halting the processing of the string with an error. (See Section 3.9, Unicode Encoding Forms.)

#2.8 Unicode Allocation

For convenience, the encoded characters of the Unicode Standard are grouped by linguistic and functional categories, such as script or writing system. For practical reasons, there are occasional departures from this general principle, as when punctuation associated with the ASCII standard is kept together with other ASCII characters in the range U+0020..U+007E rather than being grouped with other sets of general punctuation characters. By and large, however, the code charts are arranged so that related characters can be found near each other in the charts.

Grouping encoded characters by script or other functional categories offers the additional benefit of supporting various space-saving techniques in actual implementations, as for building tables or fonts.

For more information on writing systems, see Section 6.1, Writing Systems.

#2.8.1 Planes

The Unicode codespace consists of the single range of numeric values from 0 to 10FFFF₁₆, but in practice it has proven convenient to think of the codespace as divided up into planes of characters—each plane consisting of 64K code points. Because of these numeric conventions, the Basic Multilingual Plane is occasionally referred to as Plane 0. The last four hexadecimal digits in each code point indicate a character’s position inside a plane. The remaining digits indicate the plane. For example, U+23456 CJK UNIFIED IDEOGRAPH-23456 is found at location 3456₁₆ in Plane 2.

#Basic Multilingual Plane

The Basic Multilingual Plane (BMP, or Plane 0) contains the common-use characters for all the modern scripts of the world as well as many historical and rare characters. By far the majority of all Unicode characters for almost all textual data can be found in the BMP.

#Supplementary Multilingual Plane

The Supplementary Multilingual Plane (SMP, or Plane 1) is dedicated to the encoding of characters for scripts or symbols which either could not be fit into the BMP or see very infrequent usage. This includes many historic scripts, a number of lesser-used contemporary scripts, special-purpose invented scripts, notational systems or large pictographic symbol sets, and occasionally historic extensions of scripts whose core sets are encoded on the BMP.

Examples include Gothic (historic), Shavian (special-purpose invented), Musical Symbols (notational system), Domino Tiles (pictographic), and Ancient Greek Numbers (historic extension for Greek). A number of scripts, whether of historic and contemporary use, do not yet have their characters encoded in the Unicode Standard. The majority of scripts currently identified for encoding will eventually be allocated in the SMP. As a result, some areas of the SMP will experience common, frequent usage.

#Additional Ideographic Planes

The Supplementary Ideographic Plane (SIP, or Plane 2) and Tertiary Ideographic Plane (TIP, or Plane 3) are intended as an additional allocation areas for those CJK characters that could not be fit in the blocks set aside for more common CJK characters in the BMP. While there are a small number of common-use CJK characters in the SIP (for example, for Cantonese usage), the vast majority of Plane 2 characters are extremely rare or of historical interest only.

#Supplementary Special-purpose Plane

The Supplementary Special-purpose Plane (SSP, or Plane 14) is the spillover allocation area for format control characters that do not fit into the small allocation areas for format control characters in the BMP.

#Private Use Planes

The two Private Use Planes (Planes 15 and 16) are allocated for private use. Those two planes contain a total of 131,068 private-use characters to supplement the 6,400 private-use characters located in the BMP.

#2.8.2 Allocation Areas and Blocks

#Allocation Areas

The Unicode Standard does not have any normatively defined concept of areas or zones for the BMP (or other planes), but it is often handy to refer to the allocation areas of the BMP by the general types of the characters they include. These areas are merely a rough organizational device and do not restrict the types of characters that may end up being allocated in them. The description and ranges of areas may change from version to version of the standard as more new scripts, symbols, and other characters are encoded in previously reserved ranges.

#Blocks

The various allocation areas are, in turn, divided up into character blocks (see D10b in Section 3.4, Characters and Encoding), which are normatively defined, and which are used to structure the actual code charts. For a complete listing of the normative blocks in the Unicode Standard, see Blocks.txt in the Unicode Character Database.

The normative status of blocks should not, however, be taken as indicating that they define significant sets of characters. For the most part, the blocks serve only as ranges to divide up the code charts and do not necessarily imply anything else about the types of characters found in the block. Block identity cannot be taken as a reliable guide to the source, use, or properties of characters, for example, and it cannot be reliably used alone to process characters. In particular:

• Blocks are simply named ranges, and many contain reserved code points.
• Characters used in a single writing system may be found in several different blocks. For example, characters used for letters for Latin-based writing systems are found in at least 14 different blocks: Basic Latin, Latin-1 Supplement, Latin Extended-A, Latin Extended-B, Latin Extended-C, Latin Extended-D, Latin Extended-E, IPA Extensions, Phonetic Extensions, Phonetic Extensions Supplement, Latin Extended Additional, Spacing Modifier Letters, Combining Diacritical Marks, and Combining Diacritical Marks Supplement.
• Characters in a block may be used with different writing systems. For example, the danda character is encoded in the Devanagari block but is used with numerous other scripts; Arabic combining marks in the Arabic block are used with the Syriac script; and so on.
• Block definitions are not at all exclusive. For instance, many mathematical operator characters are not encoded in the Mathematical Operators block—and are not even in any block containing “Mathematical” in its name; many currency symbols are not found in the Currency Symbols block, and so on.

For reliable specification of the properties of characters, one should instead turn to the detailed, character-by-character property assignments available in the Unicode Character Database. See also Chapter 4, Character Properties. For further discussion of the relationship between the blocks in the Unicode standard and significant property assignments and sets of characters, see Unicode Standard Annex #24, “Unicode Script Property,” and Unicode Technical Standard #18, “Unicode Regular Expressions.”

#Allocation Order

The allocation order of various scripts and other groups of characters reflects the historical evolution of the Unicode Standard. While there is a certain geographic sense to the ordering of the allocation areas for the scripts, this is only a very loose correlation.

#Roadmap for Future Allocation

The unassigned ranges in the Unicode codespace will be filled with future script or symbol encodings on a space-available basis. The relevant character encoding committees follow an organized roadmap to help them decide where to encode new scripts and other characters within the available space. Until the characters are actually standardized, however, there are no absolute guarantees where future allocations will occur. In general, implementations should not make assumptions about where future scripts or other sets of symbols may be encoded based solely on the identity of neighboring blocks of characters already encoded.

See Appendix B.3, Other Unicode Online Resources for information about the roadmap and about the pipeline of approved characters in process for future publication.

#2.8.3 Assignment of Code Points

Code points in the Unicode Standard are assigned using the following guidelines:

• Where there is a single accepted standard for a script, the Unicode Standard generally follows it for the relative order of characters within that script.
• The first 256 codes follow precisely the arrangement of ISO/IEC 8859-1 (Latin 1), of which 7-bit ASCII (ISO/IEC 646 IRV) accounts for the first 128 code positions.
• Characters with common characteristics are located together contiguously. For example, the primary Arabic character block was modeled after ISO/IEC 8859-6. The Arabic script characters used in Persian, Urdu, and other languages, but not included in ISO/IEC 8859-6, are allocated after the primary Arabic character block. Right-to-left scripts are grouped together.
• In most cases, scripts with fewer than 128 characters are allocated so as not to cross 128-code-point boundaries (that is, they fit in ranges nn00..nn7F or nn80..nnFF). For supplementary characters, an additional constraint not to cross 1,024-code-point boundaries is applied (that is, scripts fit in ranges nn000..nn3FF, nn400..nn7FF, nn800..nnBFF, or nnC00..nnFFF). Such constraints enable better optimizations for tasks such as building tables for access to character properties.
• Codes that represent letters, punctuation, symbols, and diacritics that are generally shared by multiple languages or scripts are grouped together in several locations.
• The Unicode Standard does not correlate character code allocation with language-dependent collation or case. For more information on collation order, see Unicode Technical Standard #10, “Unicode Collation Algorithm.”
• Unified CJK ideographs are laid out in multiple blocks, each of which is arranged according to the Han ideograph arrangement defined in Section 18.1, Han. This ordering is roughly based on a radical-stroke count order.

#2.9 Details of Allocation

This section provides a more detailed summary of the way characters are allocated in the Unicode Standard. Figure 2-13 gives an overall picture of the allocation areas of the Unicode Standard, with an emphasis on the identities of the planes. The following subsections discuss the allocation details for specific planes.

#2.9.1 Plane 0 (BMP)

Figure 2-14 shows the Basic Multilingual Plane (BMP) in an expanded format to illustrate the allocation substructure of that plane in more detail. This section describes each allocation area, in the order of their location on the BMP.

#ASCII and Latin-1 Compatibility Area

For compatibility with the ASCII and ISO 8859-1, Latin-1 standards, this area contains the same repertoire and ordering as Latin-1. Accordingly, it contains the basic Latin alphabet, European digits, and then the same collection of miscellaneous punctuation, symbols, and additional Latin letters as are found in Latin-1.

#General Scripts Area

The General Scripts Area contains a large number of modern-use scripts of the world, including Latin, Greek, Cyrillic, Arabic, and so on. Most of the characters encoded in this area are graphic characters. A subrange of the General Scripts Area is set aside for right-to-left scripts, including Hebrew, Arabic, Thaana, and N’Ko.

#Punctuation and Symbols Area

This area is devoted mostly to all kinds of symbols, including many characters for use in mathematical notation. It also contains general punctuation, as well as most of the important format control characters.

#Supplementary General Scripts Area

This area contains scripts or extensions to scripts that did not fit in the General Scripts Area itself. It contains the Glagolitic, Coptic, and Tifinagh scripts, plus extensions for the Latin, Cyrillic, Georgian, and Ethiopic scripts.

#CJK Miscellaneous Area

The CJK Miscellaneous Area contains some East Asian scripts, such as Hiragana and Katakana for Japanese, punctuation typically used with East Asian scripts, lists of CJK radical symbols, and a large number of East Asian compatibility characters.

#CJKV Ideographs Area

This area contains almost all the unified Han ideographs in the BMP. It is subdivided into a block for the Unified Repertoire and Ordering (the initial block of 20,902 unified Han ideographs plus a small number of later additions) and another block containing Extension A (an additional 6,582 unified Han ideographs).

#General Scripts Area (Asia and Africa)

This area contains numerous blocks for additional scripts of Asia and Africa, such as Yi, Cham, Vai, and Bamum. It also contains more spillover blocks with additional characters for the Latin, Devanagari, Myanmar, and Hangul scripts.

#Hangul Area

This area consists of one large block containing 11,172 precomposed Hangul syllables, and one small block with additional, historic Hangul jamo extensions.

#Surrogates Area

The Surrogates Area contains only surrogate code points and no encoded characters. See Section 23.6, Surrogates Area, for more details.

#Private Use Area

The Private Use Area in the BMP contains 6,400 private-use characters.

#Compatibility and Specials Area

This area contains many compatibility variants of characters from widely used corporate and national standards that have other representations in the Unicode Standard. For example, it contains Arabic presentation forms, whereas the basic characters for the Arabic script are located in the General Scripts Area. The Compatibility and Specials Area also contains twelve CJK unified ideographs, a few important format control characters, the basic variation selectors, and other special characters. See Section 23.8, Specials, for more details.

#2.9.2 Plane 1 (SMP)

Figure 2-15 shows Plane 1, the Supplementary Multilingual Plane (SMP), in expanded format to illustrate the allocation substructure of that plane in more detail.

#General Scripts Areas

These areas contain a large number of historic scripts, as well as a few regional scripts which have some current use. The first of these areas also contains a small number of symbols and numbers associated with ancient scripts.

#General Scripts Areas (RTL)

There are two subranges in the SMP which are set aside for historic right-to-left scripts, such as Phoenician, Kharoshthi, and Avestan. The second of these also defaults to Bidi_Class = R and is reserved for the encoding of other historic right-to-left scripts or symbols.

#Cuneiform and Hieroglyphic Area

This area contains three large, ancient scripts: Sumero-Akkadian Cuneiform, Egyptian Hieroglyphs, and Anatolian Hieroglyphs. Other large hieroglyphic and pictographic scripts will be allocated in this area in the future.

#Ideographic Scripts Area

This area is set aside for large, historic siniform (but non-Han) logosyllabic scripts such as Tangut, Jurchen, and Khitan, and other East Asian logosyllabic scripts such as Naxi. As of Unicode 12.0, this area contains a large set of Tangut ideographs and components, the Nüshu script, and a large set of hentaigana (historic, variant form kana) characters.

#Symbols Areas

The first of these SMP Symbols Areas contains sets of symbols for notational systems, such as musical symbols, shorthands, and mathematical alphanumeric symbols. The second contains various game symbols, and large sets of miscellaneous symbols and pictographs, mostly used in compatibility mapping of East Asian character sets. Notable among these are emoji and emoticons.

#2.9.3 Plane 2 (SIP)

Plane 2, the Supplementary Ideographic Plane (SIP), consists primarily of one big area, starting from the first code point in the plane, that is dedicated to encoding additional unified CJK characters. A much smaller area, toward the end of the plane, is dedicated to additional CJK compatibility ideographic characters—which are basically just duplicated character encodings required for round-trip conversion to various existing legacy East Asian character sets. The CJK compatibility ideographic characters in Plane 2 are currently all dedicated to round-trip conversion for the CNS standard and are intended to supplement the CJK compatibility ideographic characters in the BMP, a smaller number of characters dedicated to round-trip conversion for various Korean, Chinese, and Japanese standards.

#2.9.4 Plane 3 (TIP)

Plane 3, the Tertiary Ideographic Plane (TIP), has a large area, starting from the first code point in the plane, that is dedicated to encoding additional unified CJK characters. This plane may also allocate blocks for historic precursors to the Han script—most notably the Seal Script.

#2.9.5 Other Planes

The first 4,096 code positions on Plane 14 form an area set aside for special characters that have the Default_Ignorable_Code_Point property. A small number of tag characters, plus some supplementary variation selection characters, have been allocated there. All remaining code positions on Plane 14 are reserved for future allocation of other special-purpose characters.

Plane 15 and Plane 16 are allocated for private use. Those two planes contain a total of 131,068 private-use characters, to supplement the 6,400 private-use characters located in the BMP.

All other planes are reserved; there are no characters assigned in them. The last two code positions of all planes are permanently set aside as noncharacters. (See Section 2.13, Special Characters).

#2.10 Writing Direction

Individual writing systems have different conventions for arranging characters into lines on a page or screen. Such conventions are referred to as a script’s directionality. For example, in the Latin script, characters are arranged horizontally from left to right to form lines, and lines are arranged from top to bottom, as shown in the first example of Figure 2-16.

#Bidirectional

In most Semitic scripts such as Hebrew and Arabic, characters are arranged from right to left into lines, although digits run the other way, making the scripts inherently bidirectional, as shown in the second example in Figure 2-16. In addition, left-to-right and right-to-left scripts are frequently used together. In all such cases, arranging characters into lines becomes more complex. The Unicode Standard defines an algorithm to determine the layout of a line, based on the inherent directionality of each character, and supplemented by a small set of directional controls. See Unicode Standard Annex #9, “Unicode Bidirectional Algorithm,” for more information.

#Vertical

East Asian scripts are frequently written in vertical lines in which characters are arranged from top to bottom. Lines are arranged from right to left, as shown in the third example in Figure 2-16. Such scripts may also be written horizontally, from left to right. Most East Asian characters have the same shape and orientation when displayed horizontally or vertically, but many punctuation characters change their shape when displayed vertically. In a vertical context, letters and words from other scripts are generally rotated through 90-degree angles so that they, too, read from top to bottom. Unicode Technical Report #50, “Unicode Vertical Text Layout,” defines a character property which is useful in determining the correct orientation of characters when laid out vertically in text.

In contrast to the bidirectional case, the choice to lay out text either vertically or horizontally is treated as a formatting style. Therefore, the Unicode Standard does not provide directionality controls to specify that choice.

Mongolian is usually written from top to bottom, with lines arranged from left to right, as shown in the fourth example. When Mongolian is written horizontally, the characters are rotated.

#Boustrophedon

Early Greek used a system called boustrophedon (literally, “ox-turning”). In boustrophedon writing, characters are arranged into horizontal lines, but the individual lines alternate between right to left and left to right, the way an ox goes back and forth when plowing a field, as shown in the fifth example. The letter images are mirrored in accordance with the direction of each individual line.

#Other Historical Directionalities

Other script directionalities are found in historical writing systems. For example, some ancient Numidian texts are written from bottom to top, and Egyptian hieroglyphics can be written with varying directions for individual lines.

The historical directionalities are of interest almost exclusively to scholars intent on reproducing the exact visual content of ancient texts. The Unicode Standard does not provide direct support for them. Fixed texts can, however, be written in boustrophedon or in other directional conventions by using hard line breaks and directionality overrides or the equivalent markup.

#2.11 Combining Characters

#Combining Characters

In concept, a combining character is a mark of some kind intended to be positioned relative to some other character, which is referred to as its associated base character. In the code charts, combining characters are depicted with an associated dotted circle, which stands in for the base. The visible mark for the combining character may appear above, below, to either side, crossing through, or even surrounding the dotted circle. When rendered, the glyphs that depict these characters are intended to be positioned relative to the glyph depicting the preceding base character in some combination.

The Unicode Standard distinguishes three types of combining characters: spacing, nonspacing, and enclosing. Nonspacing combining characters do not occupy a spacing position by themselves. Nevertheless, the combination of a base character and a nonspacing character may have a different advance width than the base character by itself. For example, an [î] may be slightly wider than a plain [i]. The detailed classification of which combining characters are spacing, nonspacing, or enclosing is provided in the Unicode Character Database.

All combining characters can be applied to any base character and can, in principle, be used with any script. As with other characters, the allocation of a combining character to one block or another identifies only its primary usage; it is not intended to define or limit the range of characters to which it may be applied. In the Unicode Standard, all sequences of character codes are permitted.

This does not create an obligation on implementations to support all possible combinations equally well. Thus, while application of an Arabic annotation mark to a Han character or a Devanagari consonant is permitted, it is unlikely to be supported well in rendering or to make much sense.

#Diacritics

Diacritics are the principal class of nonspacing combining characters used with the Latin, Greek, and Cyrillic scripts and their relatives. In the Unicode Standard, the term “diacritic” is defined very broadly to include accents as well as other nonspacing marks.

#Symbol Diacritics

Some diacritical marks are applied primarily to symbols. These combining marks are allocated in the Combining Diacritical Marks for Symbols block, to distinguish them from diacritical marks applied primarily to letters.

#Enclosing Combining Marks

Figure 2-17 shows examples of combining enclosing marks for symbols. The combination of an enclosing mark with a base character has the appearance of a symbol. As discussed in “Properties” later in this section, it is best to limit the use of combining enclosing marks to characters that represent symbols. This limitation minimizes the potential for surprises resulting from mismatched character properties.

A few symbol characters are intended primarily for use with enclosing combining marks. For example, U+2621 CAUTION SIGN is a winding road symbol that can be used in combination with U+20E4 COMBINING ENCLOSING UPWARD POINTING TRIANGLE or U+20DF COMBINING ENCLOSING DIAMOND. However, the enclosing combining marks can also be used in combination with arbitrary symbols, as illustrated by applying U+20E0 COMBINING ENCLOSING CIRCLE BACKSLASH to U+2615 HOT BEVERAGE to create a “no drinks allowed” symbol. Furthermore, no formal restriction prevents enclosing combining marks from being used with non-symbols, as illustrated by applying U+20DD COMBINING ENCLOSING CIRCLE to U+062D ARABIC LETTER HAH to represent a circled hah.

#Script-Specific Combining Characters

Some scripts, such as Hebrew, Arabic, and the scripts of India and Southeast Asia, have both spacing and nonspacing combining characters specific to those scripts. Many of these combining characters encode vowel letters. As such, they are not generally referred to as diacritics, but may have script-specific terminology such as harakat (Arabic) or matra (Devanagari). See Section 7.9, Combining Marks.

#2.11.1 Sequence of Base Characters and Combining Characters

In the Unicode Standard, all combining characters are to be used in sequence following the base characters to which they apply. The sequence of Unicode characters <U+0061 “a” LATIN SMALL LETTER A, U+0308 “ ̈”COMBINING DIAERESIS, U+0075 “u” LATIN SMALL LETTER U> unambiguously represents “äu” and not “aü”, as shown in Figure 2-18.

#Ordering

The ordering convention used by the Unicode Standard—placing combining marks after the base character to which they apply—is consistent with the logical order of combining characters in Semitic and Indic scripts, the great majority of which (logically or phonetically) follow the base characters with which they are associated. This convention also conforms to the way modern font technology handles the rendering of nonspacing graphical forms (glyphs), so that mapping from character memory representation order to font rendering order is simplified. It is different from the convention used in the bibliographic standard ISO 5426.

#Indic Vowel Signs

Some Indic vowel signs are rendered to the left of a consonant letter or consonant cluster, even though their logical order in the Unicode encoding follows the consonant letter. In the charts, these vowels are depicted to the left of dotted circles (see Figure 2-19). The coding of these vowels in pronunciation order and not in visual order is consistent with the ISCII standard.

#Properties

A sequence of a base character plus one or more combining characters generally has the same properties as the base character. For example, “A” followed by “ˆ” has the same properties as “”. For this reason, most Unicode algorithms ensure that such sequences behave the same way as the corresponding base character. However, when the combining character is an enclosing combining mark—in other words, when its General_Category value is Me—the resulting sequence has the appearance of a symbol. In Figure 2-20, enclosing the exclamation mark with U+20E4 COMBINING ENCLOSING UPWARD POINTING TRIANGLE produces a sequence that looks like U+26A0 WARNING SIGN.

Because the properties of U+0021 EXCLAMATION MARK are that of a punctuation character, they are different from those of U+26A0 WARNING SIGN. For example, the two will behave differently for line breaking. To avoid unexpected results, it is best to limit the use of combining enclosing marks to characters that encode symbols. For that reason, the warning sign is separately encoded as a miscellaneous symbol in the Unicode Standard and does not have a decomposition.

#2.11.2 Multiple Combining Characters

In some instances, more than one combining character is applied to a single base character (see Figure 2-21). The Unicode Standard does not restrict the number of combining characters that may follow a base character. The following discussion summarizes the default treatment of multiple combining characters. (For further discussion, see Section 3.6, Combination.)

If the combining characters can interact typographically—for example, U+0304 COMBINING MACRON and U+0308 COMBINING DIAERESIS—then the order of graphic display is determined by the order of coded characters (see Table 2-5). By default, the diacritics or other combining characters are positioned from the base character’s glyph outward. Combining characters placed above a base character will be stacked vertically, starting with the first encountered in the logical store and continuing for as many marks above as are required by the character codes following the base character. For combining characters placed below a base character, the situation is reversed, with the combining characters starting from the base character and stacking downward. For combining characters placed to the left of a base character, the combining characters start from the base character and are arranged leftward. For combining characters placed to the right of a base character, the combining characters start from the base character and are arranged rightward.

When combining characters do not interact typographically, the relative ordering of contiguous combining marks cannot result in any visual distinction and thus is insignificant.

Glyph	Equivalent Sequences
ã	LATIN SMALL LETTER A WITH TILDE
	LATIN SMALL LETTER A + COMBINING TILDE
ȧ	LATIN SMALL LETTER A WITH DOT ABOVE
	LATIN SMALL LETTER A + COMBINING DOT ABOVE
ạ̃	LATIN SMALL LETTER A WITH TILDE + COMBINING DOT BELOW
	LATIN SMALL LETTER A + COMBINING TILDE + COMBINING DOT BELOW
	LATIN SMALL LETTER A WITH DOT BELOW + COMBINING TILDE
	LATIN SMALL LETTER A + COMBINING DOT BELOW + COMBINING TILDE
ȧ̇	LATIN SMALL LETTER A WITH DOT BELOW + COMBINING DOT ABOVE
	LATIN SMALL LETTER A + COMBINING DOT BELOW + COMBINING DOT ABOVE
	LATIN SMALL LETTER A WITH DOT ABOVE + COMBINING DOT BELOW
	LATIN SMALL LETTER A + COMBINING DOT ABOVE + COMBINING DOT BELOW
ấ	LATIN SMALL LETTER A WITH CIRCUMFLEX AND ACUTE
	LATIN SMALL LETTER A WITH CIRCUMFLEX + COMBINING ACUTE ACCENT
	LATIN SMALL LETTER A + COMBINING CIRCUMFLEX ACCENT + COMBINING ACUTE ACCENT
á̂	LATIN SMALL LETTER A WITH ACUTE + COMBINING CIRCUMFLEX ACCENT
	LATIN SMALL LETTER A + COMBINING ACUTE ACCENT + COMBINING CIRCUMFLEX ACCENT

Another example of multiple combining characters above the base character can be found in Thai, where a consonant letter can have above it one of the vowels U+0E34 through U+0E37 and, above that, one of four tone marks U+0E48 through U+0E4B. The order of character codes that produces this graphic display is base consonant character + vowel character +tone mark character, as shown in Figure 2-21.

Many combining characters have specific typographical traditions that provide detailed rules for the expected rendering. These rules override the default stacking behavior. For example, certain combinations of combining marks are sometimes positioned horizontally rather than stacking or by ligature with an adjacent nonspacing mark (see Table 2-6). When positioned horizontally, the order of codes is reflected by positioning in the predominant direction of the script with which the codes are used. For example, in a left-to-right script, horizontal accents would be coded from left to right. In Table 2-6, the top example is correct and the bottom example is incorrect.

Such override behavior is associated with specific scripts or alphabets. For example, when used with the Greek script, the “breathing marks” U+0313 COMBINING COMMA ABOVE (psili) and U+0314 COMBINING REVERSED COMMA ABOVE (dasia) require that, when used together with a following acute or grave accent, they be rendered side-by-side rather than the accent marks being stacked above the breathing marks. The order of codes here is base character code + breathing mark code + accent mark code. This example demonstrates the script-dependent or writing-system-dependent nature of rendering combining diacritical marks.

Glyph	Sequence	Note
ἄ	GREEK SMALL LETTER ALPHA + COMBINING COMMA ABOVE (psili) + COMBINING ACUTE ACCENT (oxia)	This is correct
ά̓	GREEK SMALL LETTER ALPHA + COMBINING ACUTE ACCENT (oxia) + COMBINING COMMA ABOVE (psili)	This is incorrect

For additional examples of script-specific departure from default stacking of sequences of combining marks, see the discussion about the positioning of multiple points and marks in Section 9.1, Hebrew, the discussion of nondefault placement of Arabic vowel marks accompanying Figure 9-5 in Section 9.2, Arabic, or the discussion of horizontal combination of titlo letters in Old Church Slavonic accompanying Figure 7-6 in Section 7.4, Cyrillic.

Vietnamese is written with the Latin script, and regularly uses multiple diacritics on single letters when displaying tone marks above or below vowel letters that already contain diacritic marks. Acute or grave tone marks are often seen offset horizontally to the right or left of a circumflex mark above a vowel letter. These conventions are shown, for example, in the code chart font used for the precomposed Vietnamese vowel plus tone mark combinations in the code charts in the range U+1EA0..U+1EF9 in the Latin Extended Additional block. However, the exact placement of tone marks in Vietnamese text is considered stylistic, and often varies considerably from font to font.

For other types of nondefault stacking behavior, see the discussion about the positioning of combining parentheses in the subsection “Combining Diacritical Marks Extended: U+1AB0–U+1AFF” in Section 7.9, Combining Marks.

The Unicode Standard specifies default stacking behavior to offer guidance about which character codes are to be used in which order to represent the text, so that texts containing multiple combining marks can be interchanged reliably. The Unicode Standard does not aim to regulate or restrict typographical tradition. Fonts and rendering systems should be designed to override default stacking of multiple combining marks where necessary to produce the correct appearance for the relevant script and language context.

#2.11.3 Ligated Multiple Base Characters

When the glyphs representing two base characters merge to form a ligature, the combining characters must be rendered correctly in relation to the ligated glyph (see Figure 2-22). Internally, the software must distinguish between the nonspacing marks that apply to positions relative to the first part of the ligature glyph and those that apply to the second part. (For a discussion of general methods of positioning nonspacing marks, see Section 5.12, Strategies for Handling Nonspacing Marks.)

For more information, see “Application of Combining Marks” in Section 3.6, Combination.

Ligated base characters with multiple combining marks do not commonly occur in most scripts. However, in some scripts, such as Arabic, this situation occurs quite often when vowel marks are used. It arises because of the large number of ligatures in Arabic, where each element of a ligature is a consonant, which in turn can have a vowel mark attached to it. Ligatures can even occur with three or more characters merging; vowel marks may be attached to each part.

#2.11.4 Exhibiting Nonspacing Marks in Isolation

Nonspacing combining marks used by the Unicode Standard may be exhibited in apparent isolation by applying them to U+00A0 NO-BREAK SPACE. This convention might be employed, for example, when talking about the combining mark itself as a mark, rather than using it in its normal way in text (that is, applied as an accent to a base letter or in other combinations).

Prior to Version 4.1 of the Unicode Standard, the standard recommended the use of U+0020 SPACE for display of isolated combining marks. This practice is no longer recommended because of potential conflicts with the handling of sequences of U+0020 SPACE characters in such contexts as XML. For additional ways of displaying some diacritical marks, see “Spacing Clones of Diacritical Marks” in Section 7.9, Combining Marks.

#2.11.5 “Characters” and Grapheme Clusters

End users have various concepts about what constitutes a letter or “character” in the writing system for their language or languages. The precise scope of these end-user “characters” depends on the particular written language and the orthography it uses. In addition to the many instances of accented letters, they may extend to digraphs such as Slovak “ch”, trigraphs or longer combinations, and sequences using spacing letter modifiers, such as “k^w”. Such concepts are often important for processes such as collation, for the definition of characters in regular expressions, and for counting “character” positions within text. In instances such as these, what the user thinks of as a character may affect how the collation or regular expression will be defined or how the “characters” will be counted. Words and other higher-level text elements generally do not split within elements that a user thinks of as a character, even when the Unicode representation of them may consist of a sequence of encoded characters.

The variety of these end-user-perceived characters is quite great—particularly for digraphs, ligatures, or syllabic units. Furthermore, it depends on the particular language and writing system that may be involved. Despite this variety, however, the core concept “characters that should be kept together” can be defined for the Unicode Standard in a language-independent way. This core concept is known as a grapheme cluster. A grapheme cluster consists of a base character followed by any number of continuing characters, where a continuing character may include any nonspacing combining mark, certain spacing combining marks, or a join control.

An implementation operating on such a cluster would almost never want to break between its elements for rendering, editing, or other such text processes; the grapheme cluster is treated as a single unit. Unicode Standard Annex #29, “Unicode Text Segmentation,” provides a complete formal definition of a grapheme cluster and discusses its application in the context of editing and other text processes. Implementations also may tailor the definition of a grapheme cluster, so that under limited circumstances, particular to one written language or another, the grapheme cluster may more closely pertain to what end users think of as “characters” for that language.

#2.12 Equivalent Sequences

In cases involving two or more sequences considered to be equivalent, the Unicode Standard does not prescribe one particular sequence as being the correct one; instead, each sequence is merely equivalent to the others. Figure 2-23 illustrates the two major forms of equivalent sequences formally defined by the Unicode Standard. In the first example, the sequences are canonically equivalent. Both sequences should display and be interpreted the same way. The second and third examples illustrate different compatibility sequences. Compatible-equivalent sequences may have format differences in display and may be interpreted differently in some contexts.

If an application or user attempts to distinguish between canonically equivalent sequences, as shown in the first example in Figure 2-23, there is no guarantee that other applications would recognize the same distinctions. To prevent the introduction of interoperability problems between applications, such distinctions must be avoided wherever possible. Making distinctions between compatibly equivalent sequences is less problematical. However, in restricted contexts, such as the use of identifiers, avoiding compatibly equivalent sequences reduces possible security issues. See Unicode Technical Report #36, “Unicode Security Considerations.”

#2.12.1 Normalization

Where a unique representation is required, a normalized form of Unicode text can be used to eliminate unwanted distinctions. The Unicode Standard defines four normalization forms: Normalization Form D (NFD), Normalization Form KD (NFKD), Normalization Form C (NFC), and Normalization Form KC (NFKC). Roughly speaking, NFD and NFKD decompose characters where possible, while NFC and NFKC compose characters where possible. For more information, see Unicode Standard Annex #15, “Unicode Normalization Forms,” and Section 3.11, Normalization Forms.

A key part of normalization is to provide a unique canonical order for visually nondistinct sequences of combining characters. Figure 2-24 shows the effect of canonical ordering for multiple combining marks applied to the same base character.

In the first row of Figure 2-24, the two sequences are visually nondistinct and, therefore, equivalent. The sequence on the right has been put into canonical order by reordering in ascending order of the Canonical_Combining_Class (ccc) values. The ccc values are shown below each character. The second row of Figure 2-24 shows an example where combining marks interact typographically—the two sequences have different stacking order, and the order of combining marks is significant. Because the two combining marks have been given the same combining class, their ordering is retained under canonical reordering. Thus the two sequences in the second row are not equivalent.

#2.12.2 Decompositions

Precomposed characters are formally known as decomposables, because they have decompositions to one or more other characters. There are two types of decompositions:

• Canonical. The character and its decomposition should be treated as essentially equivalent.
• Compatibility. The decomposition may remove some information (typically formatting information) that is important to preserve in particular contexts.

#Types of Decomposables

Conceptually, a decomposition implies reducing a character to an equivalent sequence of constituent parts, such as mapping an accented character to a base character followed by a combining accent. The vast majority of nontrivial decompositions are indeed a mapping from a character code to a character sequence. However, in a small number of exceptional cases, there is a mapping from one character to another character, such as the mapping from ohm to capital omega. Finally, there are the “trivial” decompositions, which are simply a mapping of a character to itself. They are really an indication that a character cannot be decomposed, but are defined so that all characters formally have a decomposition. The definition of decomposable is written to encompass only the nontrivial types of decompositions; therefore these characters are considered nondecomposable.

In summary, three types of characters are distinguished based on their decomposition behavior:

• Canonical decomposable. A character that is not identical to its canonical decomposition.
• Compatibility decomposable. A character whose compatibility decomposition is not identical to its canonical decomposition.
• Nondecomposable. A character that is identical to both its canonical decomposition and its compatibility decomposition. In other words, the character has trivial decompositions (decompositions to itself). Loosely speaking, these characters are said to have “no decomposition,” even though, for completeness, the algorithm that defines decomposition maps such characters to themselves.

Because of the way decompositions are defined, a character cannot have a nontrivial canonical decomposition while having a trivial compatibility decomposition. Characters with a trivial compatibility decomposition are therefore always nondecomposables.

#Examples

Figure 2-25 illustrates these three types. Compatibility decompositions that are redundant because they are identical to the canonical decompositions are not shown.

The figure illustrates two important points:

• Decompositions may be to single characters or to sequences of characters. Decompositions to a single character, also known as singleton decompositions, are seen for the ohm sign and the halfwidth katakana ka in Figure 2-25. Because of examples like these, decomposable characters in Unicode do not always consist of obvious, separate parts; one can know their status only by examining the data tables for the standard.
• A very small number of characters are both canonical and compatibility decomposable. The example shown in Figure 2-25 is for the Greek hooked upsilon symbol with an acute accent. It has a canonical decomposition to one sequence and a compatibility decomposition to a different sequence.

For more precise definitions of these terms, see Chapter 3, Conformance.

#2.12.3 Non-decomposition of Certain Diacritics

Most characters that one thinks of as being a letter “plus accent” have formal decompositions in the Unicode Standard. For example, see the canonical decomposable U+00C1 LATIN CAPITAL LETTER A WITH ACUTE shown in Figure 2-25. There are, however, exceptions involving certain types of diacritics and other marks.

#Overlaid and Attached Diacritics

Based on the pattern for accented letters, implementers often also expect to encounter formal decompositions for characters which use various overlaid diacritics such as slashes and bars to form new Latin (or Cyrillic) letters. For example, one might expect a decomposition for U+00D8 LATIN CAPITAL LETTER O WITH STROKE involving U+0338 COMBINING LONG SOLIDUS OVERLAY. However, such decompositions involving overlaid diacritics are not formally defined in the Unicode Standard.

For historical and implementation reasons, there are no decompositions for characters with overlaid diacritics such as slashes and bars, nor for most diacritic hooks, swashes, tails, and other similar modifications to the graphic form of a base character. In such cases, the generic identification of the overlaid element is not specific enough to identify which part of the base glyph is to be overlaid. The characters involved include prototypical overlaid diacritic letters as U+0268 LATIN SMALL LETTER I WITH STROKE, but also characters with hooks and descenders, such as U+0188 LATIN SMALL LETTER C WITH HOOK, U+049B CYRILLIC SMALL LETTER KA WITH DESCENDER, and U+0499 CYRILLIC SMALL LETTER ZE WITH DESCENDER.

There are three exceptional attached diacritics which are regularly decomposed, namely U+0327 COMBINING CEDILLA, U+0328 COMBINING OGONEK, and U+031B COMBINING HORN (which is used in Vietnamese letters).

#Other Diacritics

There are other characters for which the name and glyph appear to imply the presence of a decomposable diacritic, but which have no decomposition defined in the Unicode Standard. A prominent example is the Pashto letter U+0681 ARABIC LETTER HAH WITH HAMZA ABOVE. In these cases, as for the overlaid diacritics, the composed character and the sequence of base letter plus combining diacritic are not equivalent, although their renderings would be very similar. See the text on “Combining Hamza Above” in Section 9.2, Arabic for further complications related to this and similar characters.

#Character Names and Decomposition

One cannot determine the decomposition status of a Latin letter from its Unicode name, despite the existence of phrases such as “...WITH ACUTE” or “...WITH STROKE”. The normative decomposition mappings listed in the Unicode Character Database are the only formal definition of decomposition status.

#Simulated Decomposition in Processing

Because the Unicode characters with overlaid diacritics or similar modifications to their base form shapes have no formal decompositions, some kinds of text processing that would ordinarily use Normalization Form D (NFD) internally to separate base letters from accents may end up simulating decompositions instead. Effectively, this processing treats overlaid diacritics as if they were represented by a separately encoded combining mark. For example, a common operation in searching or matching is to sort (or match) while ignoring accents and diacritics on letters. This is easy to do with characters that formally decompose; the text is decomposed, and then the combining marks for the accents are ignored. However, for letters with overlaid diacritics, the effect of ignoring the diacritic has to be simulated instead with data tables that go beyond simple use of Unicode decomposition mappings.

#Security Issue

The lack of formal decompositions for characters with overlaid diacritics means that there are increased opportunities for spoofing involving such characters. The display of a base letter plus a combining overlaid mark such as U+0335 COMBINING SHORT STROKE OVERLAY may look the same as the encoded base letter with bar diacritic, but the two sequences are not canonically equivalent and would not be folded together by Unicode normalization.

Implementations of writing systems which make use of letters with overlaid diacritics typically do not mix atomic representation (use of a precomposed letter with overlaid diacritic) with sequential representation (use of a sequence of base letter plus combining mark for the overlaid diacritic). Mixing these conventions is avoided precisely because the atomic representation and the sequential representation are not canonically equivalent. In most cases the atomic representation is the preferred choice, because of its convenience and more reliable display.

Security protocols for identifiers may disallow either the sequential representation or the atomic representation of a letter with an overlaid diacritic to try to minimize spoofing opportunities. However, when this is done, it is incumbent on the protocol designers first to verify whether the atomic or the sequential representation is in actual use. Disallowing the preferred convention, while instead forcing use of the unpreferred one for a particular writing system can have the unintended consequence of increasing confusion about use—and may thereby reduce the usability of the protocol for its intended purpose.

For more information and data for handling these confusable sequences involving overlaid diacritics, see Unicode Technical Report #36, “Unicode Security Considerations.”

#2.13 Special Characters

The Unicode Standard includes a small number of important characters with special behavior; some of them are introduced in this section. It is important that implementations treat these characters properly. For a list of these and similar characters, see Section 4.12, Characters with Unusual Properties; for more information about such characters, see Section 23.1, Control Codes; Section 23.2, Layout Controls; Section 23.7, Noncharacters; and Section 23.8, Specials.

#2.13.1 Special Noncharacter Code Points

The Unicode Standard contains a number of code points that are intentionally not used to represent assigned characters. These code points are known as noncharacters. They are permanently reserved for internal use and are not used for open interchange of Unicode text. For more information on noncharacters, see Section 23.7, Noncharacters.

#2.13.2 Byte Order Mark (BOM)

The UTF-16 and UTF-32 encoding forms of Unicode plain text are sensitive to the byte ordering that is used when serializing text into a sequence of bytes, such as when writing data to a file or transferring data across a network. Some processors place the least significant byte in the initial position; others place the most significant byte in the initial position. Ideally, all implementations of the Unicode Standard would follow only one set of byte order rules, but this scheme would force one class of processors to swap the byte order on reading and writing plain text files, even when the file never leaves the system on which it was created.

To have an efficient way to indicate which byte order is used in a text, the Unicode Standard contains two code points, U+FEFF ZERO WIDTH NO-BREAK SPACE (byte order mark) and U+FFFE (a noncharacter), which are the byte-ordered mirror images of each other. When a BOM is received with the opposite byte order, it will be recognized as a noncharacter and can therefore be used to detect the intended byte order of the text. The BOM is not a control character that selects the byte order of the text; rather, its function is to allow recipients to determine which byte ordering is used in a file.

#Unicode Signature

An initial BOM may also serve as an implicit marker to identify a file as containing Unicode text. For UTF-16, the sequence FE₁₆ FF₁₆ (or its byte-reversed counterpart, FF₁₆ FE₁₆) is exceedingly rare at the outset of text files that use other character encodings. The corresponding UTF-8 BOM sequence, EF₁₆ BB₁₆ BF₁₆, is also exceedingly rare. In either case, it is therefore unlikely to be confused with real text data. The same is true for both single-byte and multibyte encodings.

Data streams (or files) that begin with the U+FEFF byte order mark are likely to contain Unicode characters.

Conformance to the Unicode Standard does not require the use of the BOM as such a signature. See Section 23.8, Specials, for more information on the byte order mark and its use as an encoding signature.

#2.13.3 Layout and Format Control Characters

The Unicode Standard defines several characters that are used to control joining behavior, bidirectional ordering control, and alternative formats for display. Their specific use in layout and formatting is described in Section 23.2, Layout Controls.

#2.13.4 The Replacement Character

U+FFFD REPLACEMENT CHARACTER is the general substitute character in the Unicode Standard. It can be substituted for any “unknown” character in another encoding that cannot be mapped in terms of known Unicode characters (see Section 5.3, Unknown and Missing Characters, and Section 23.8, Specials).

#2.13.5 Control Codes

In addition to the special characters defined in the Unicode Standard for a number of purposes, the standard incorporates the legacy control codes for compatibility with the ISO/IEC 2022 framework, ASCII, and the various protocols that make use of control codes. Rather than simply being defined as byte values, however, the legacy control codes are assigned to Unicode code points: U+0000..U+001F, U+007F..U+009F. Those code points for control codes must be represented consistently with the various Unicode encoding forms when they are used with other Unicode characters. For more information on control codes, see Section 23.1, Control Codes.

#2.14 Conforming to the Unicode Standard

Conformance requirements are a set of unambiguous criteria to which a conformant implementation of a standard must adhere, so that it can interoperate with other conformant implementations. The universal scope of the Unicode Standard complicates the task of rigorously defining such conformance requirements for all aspects of the standard. Making conformance requirements overly confining runs the risk of unnecessarily restricting the breadth of text operations that can be implemented with the Unicode Standard or of limiting them to a one-size-fits-all lowest common denominator. In many cases, therefore, the conformance requirements deliberately cover only minimal requirements, falling far short of providing a complete description of the behavior of an implementation. Nevertheless, there are many core aspects of the standard for which a precise and exhaustive definition of conformant behavior is possible.

This section gives examples of both conformant and nonconformant implementation behavior, illustrating key aspects of the formal statement of conformance requirements found in Chapter 3, Conformance.

#2.14.1 Characteristics of Conformant Implementations

An implementation that conforms to the Unicode Standard has the following characteristics:

It treats characters according to the specified Unicode encoding form.

• The byte sequence <20 20> is interpreted as U+2020 ‘†’ DAGGER in the UTF-16 encoding form.
• The same byte sequence <20 20> is interpreted as the sequence of two spaces, <U+0020, U+0020>, in the UTF-8 encoding form.

It interprets characters according to the identities, properties, and rules defined for them in this standard.

• U+2423 is ‘␣’ OPEN BOX, not ‘ぃ’ hiragana small i (which is the meaning of the bytes 2423₁₆ in JIS).
• U+00F4 ‘ô’ is equivalent to U+006F ‘o’ followed by U+0302 ‘ ̂’, but not equivalent to U+0302 followed by U+006F.
• U+05D0 ‘א’ followed by U+05D1 ‘ב’ looks like ‘בא’, not ‘אב’ when displayed.

When an implementation supports the display of Arabic, Hebrew, or other right-to-left characters and displays those characters, they must be ordered according to the Bidirectional Algorithm described in Unicode Standard Annex #9, “Unicode Bidirectional Algorithm.”

When an implementation supports Arabic, Devanagari, or other scripts with complex shaping for their characters and displays those characters, at a minimum the characters are shaped according to the relevant block descriptions. (More sophisticated shaping can be used if available.)

#2.14.2 Unacceptable Behavior

It is unacceptable for a conforming implementation:

To use unassigned codes.

• U+2073 is unassigned and not usable for ‘³’ (superscript 3) or any other character.

To corrupt unsupported characters.

• U+03A1 “P” GREEK CAPITAL LETTER RHO should not be changed to U+00A1 (first byte dropped), U+0050 (mapped to Latin letter P), U+A103 (bytes reversed), or anything other than U+03A1.

To remove or alter uninterpreted code points in text that purports to be unmodified.

• U+2029 is PARAGRAPH SEPARATOR and should not be dropped by applications that do not support it.

#2.14.3 Acceptable Behavior

It is acceptable for a conforming implementation:

To support only a subset of the Unicode characters.

• An application might not provide mathematical symbols or the Thai script, for example.

To transform data knowingly.

• Uppercase conversion: ‘a’ transformed to ‘A’
• Romaji to kana: ‘kyo’ transformed to きょ
• Decomposition: U+247D ‘(10)’ decomposed to <U+0028, U+0031, U+0030, U+0029>

To build higher-level protocols on the character set.

• Examples are defining a file format for compression of characters or for use with rich text.

To define private-use characters.

• Examples of characters that might be defined for private use include additional ideographic characters (gaiji) or existing corporate logo characters.

To not support the Bidirectional Algorithm or character shaping in implementations that do not support complex scripts, such as Arabic and Devanagari.

To not support the Bidirectional Algorithm or character shaping in implementations that do not display characters, as, for example, on servers or in programs that simply parse or transcode text, such as an XML parser.

Code conversion between other character encodings and the Unicode Standard will be considered conformant if the conversion is accurate in both directions.

#2.14.4 Supported Subsets

The Unicode Standard does not require that an application be capable of interpreting and rendering all Unicode characters so as to be conformant. Many systems will have fonts for only some scripts, but not for others; sorting and other text-processing rules may be implemented for only a limited set of languages. As a result, an implementation is able to interpret a subset of characters.

The Unicode Standard provides no formalized method for identifying an implemented subset. Furthermore, such a subset is typically different for different aspects of an implementation. For example, an application may be able to read, write, and store any Unicode character and to sort one subset according to the rules of one or more languages (and the rest arbitrarily), but have access to fonts for only a single script. The same implementation may be able to render additional scripts as soon as additional fonts are installed in its environment. Therefore, the subset of interpretable characters is typically not a static concept.