[Unicode]  Technical Reports

Proposed Draft Unicode Technical Report #53


Authors Roozbeh Pournader (roozbeh@unicode.org), Bob Hallissy (bob_hallissy@sil.org), Lorna Evans (lorna_evans@sil.org)
Date 2018-03-02
This Version http://www.unicode.org/reports/tr53/tr53-2.html
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Revision 2



This technical report specifies an algorithm that can be utilized during rendering for determining correct display of Arabic combining mark sequences.

This UTR makes no change to Unicode normalization forms, and does not propose a new normalization form. Instead, this is similar to the processing used in https://docs.microsoft.com/en-us/typography/script-development/use: a transient process which is used to reorder text for display in an internal rendering pipeline. This reordering is not intended for modifying original text, nor for open interchange.


This is a draft document which may be updated, replaced, or superseded by other documents at any time. Publication does not imply endorsement by the Unicode Consortium. This is not a stable document; it is inappropriate to cite this document as other than a work in progress.

A Unicode Technical Report (UTR) contains informative material. Conformance to the Unicode Standard does not imply conformance to any UTR. Other specifications, however, are free to make normative references to a UTR.

Please submit corrigenda and other comments with the online reporting form [Feedback]. Related information that is useful in understanding this document is found in the References. For the latest version of the Unicode Standard see [Unicode]. For a list of current Unicode Technical Reports see [Reports]. For more information about versions of the Unicode Standard, see [Versions].


1 Overview

The combining classes of Arabic combining characters in Unicode are different than combining classes in most other scripts. They are a mixture of special classes for specific marks plus two more generalized classes for all the other marks. This has resulted in inconsistent and/or incorrect rendering for sequences with multiple combining marks since Unicode 2.0.

The Arabic Mark Transient Reordering Algorithm (AMTRA) described herein is the recommended solution to achieving correct and consistent rendering of Arabic combining mark sequences. This algorithm provides results that match user expectations and assures that canonically equivalent sequences are rendered identically, independent of the order of the combining marks.

2 Background

Rules and recommendations for the correct display of combining marks are discussed in a number of places in the Unicode Standard, including sections 5.13, 7.9, and 9.2 [Unicode]. Some general principles include:

In Unicode, the Arabic script combining marks include eleven different non-zero canonical combining class values, as shown in Table 1. When a combining character sequence includes marks from more than one of these classes, the rendering system has to determine a display order in which to position these marks on the base character.

While it might be tempting to just use NFC or NFD, neither of these normalization forms will yield what Arabic readers expect. For one example that will be easily understood by all readers of Arabic script, given a combining character sequence including a shadda (ccc=33) and damma (ccc=31), NFC and NFD will move the damma before the shadda – at which point the default inside-out rendering rule would place the shadda above the damma, which is incorrect.

Some cases are obvious to readers of languages written with Arabic script, and thus will likely get the same display from various rendering implementations. However, many of the combining marks, especially those with ccc=220 and ccc=230, are not commonly understood. Different rendering implementations have made different decisions regarding display order, resulting in inconsistent behavior between one system and another.

AMTRA defines a method to reorder Arabic combining marks in order to accomplish the following goals:

Table 1: Canonical combining class values for marks used in Arabic script
Canonical Combining Class (ccc) value
Combining marks in this class
0 Combining grapheme joiner
27 fathatan, open fathatan
28 dammatan, open dammatan
29 kasratan, open kasratan
30 fatha, small fatha
31 damma, small damma
32 kasra, small kasra
33 shadda
34 sukun
35 Superscript alef
220 All other below combining marks
230 All other above combining marks

3 AMTRA Details

The algorithm starts by reordering combining marks according to one of the Unicode Normalization forms, and then makes adjustments by moving certain marks closer to the base.

3.1 Modifier Combining Marks (MCM)

For use by this algorithm, we define a group of combining marks called “Modifier Combining Marks” (MCM). MCM are combining characters that are normally used to modify the base character before them, and should normally be rendered closer to the base character than tashkil (supplementary diacritics, including vowels). The MCM characters are not formally classified as ijam (consonant pointing/nukta, etc) in the Unicode Standard, but they are usually perceived by users as ijam.

The complete list of MCM characters is:


Review note: After an appropriate review period, the list of characters included in the MCM will be considered “fixed.” Adding an existing Unicode character to the list of MCM could change the rendering of data that assumes the implementation of AMTRA. If new characters are encoded in Unicode, those new characters could be added to the MCM at the time of encoding if needed.

The set of MCM characters is stable. Characters from Unicode Version 10.0 or earlier will not be added or removed from this set in future updates to this UTR. Characters added after version 10.0 may be added to MCM at the time they are incorporated into the standard but not after.


In the following specification, parenthetical definitions, for example (D56), refer to definitions in the Unicode core specification.

Input: A Combining Character Sequence (D56) containing one or more Arabic combining marks.

Output: A canonically equivalent Combining Character Sequence reordered for rendering using inside-out stacking.


  1. Normalize the input to NFD
  2. Within the result, for each maximal-length substring, S, of non-Starter (D107) characters, re-order as follows:
    1. Move any shadda characters (ccc=33) to the beginning of S.
    2. If a sequence of ccc=230 characters begins with any MCM characters, move the sequence of such MCM characters to the beginning of S (before any characters with ccc=33).
    3. If a sequence of ccc=220 characters begins with any MCM characters, move the sequence of such MCM characters to the beginning of S (before any MCM with ccc=230 or ccc=33).

4 Demonstrating AMTRA

4.1 Artificial Test Case

The following figure demonstrates the algorithm using an artificial sequence of characters:


4.2 Override Mechanism for Exceptions

The default display order implemented by the AMTRA will be correct for most uses. However in situations where a different mark order is desired, U+034F COMBINING GRAPHEME JOINER (CGJ) can be used to achieve the desired display order. The following sections give examples of the use of CGJ.

4.3 Examples

The following examples demonstrate why each of the respective characters is included in the MCM.


The use of combining hamza above and below is discussed in [Unicode, Chapter 9].

Example 1 [Quran1, page 9, end of line 5]


In Example 1, AMTRA puts a damma over a hamza above:


If an orthography needs to put the hamza above over the damma, the text should be encoded as <damma, CGJ, hamza above>:


AMTRA puts the kasra below a hamza below:


If an orthography needs to put the hamza below under the kasra, the text should be encoded as <kasra, CGJ, hamza below>:



Regarding inclusion of this mark in the MCM, Kew says “The ARABIC NASALIZATION MARK is considered equivalent to a ‘nukta’, as it is a modifier that binds tightly to the underlying letter;” (italics added for emphasis) [Kew].The character referred to by Kew as “ARABIC NASALIZATION MARK” is the character encoded as U+0658 ARABIC MARK NOON GHUNNA.


SMALL HIGH SEEN is included in MCM because most Quranic orthographies use the character as an MCM only. Orthographies that place the small seen differently will need to use a CGJ.

Example 2a [Al-Hilâlî]


Example 2b [Al-Hilâlî]


In Example 2a, the small high seen is rendered below the sukun, while in Example 2b, it is rendered over it. The examples are indeed from the same document (Al-Hilâlî and Khân 1996), just two pages away. The small high seen has different roles: in Example 2a it is a hint that the base letter, sad, should be pronounced as if it was a seen; in Example 2b, it is a pause-related hint.

Example 2a (characters and properties):


Running AMTRA on this string does not result in any changes.

Example 2b (characters and properties):


Running AMTRA on the string in Example 2b resulted in an undesired change. It puts a sukun over a seen above. If an orthography needs to put the seen above over the sukun, the text should be encoded as <sukun, CGJ, seen above>.



Example 3 [Milo, page 9, line 11]


In Example 3, AMTRA puts a shadda over a small high yeh.


If an orthography needs to put the small high yeh over the shadda, the text should be encoded as <shadda, CGJ, small high yeh>.


Running AMTRA on this string does not result in any changes.

[Pournader] notes that U+08F3 ARABIC SMALL HIGH WAW “is functionally similar to the already-encoded U+06E7 ARABIC SMALL HIGH YEH” and therefore small high waw is included in MCM.


Example 4a [Quran2]


Example 4a has a sukun over a small high noon. AMTRA puts a sukun over a small high noon. If an orthography needs to put small high noon over sukun, the text should be encoded as <sukun, CGJ, small high noon>.


Example 4b


Example 4b shows a practical orthography that uses small high noon for nasalization. It is theoretically possible for a vowel to appear above the small high noon in this practical orthography. In such a case, AMTRA puts the vowel (in this case damma) above small high noon.


In order to force the small high noon above the vowel, use the CGJ (<oe, damma, CGJ, small high noon>).


5 Supplemental Information

5.1 Use of NFD and not NFC

NFD assures that sequences such as <superscript alef, madda> always result in the same ordering, independent of the base letter. If the algorithm were to use NFC instead, the sequence <alef, superscript alef, madda> would have resulted in a different order than <lam, superscript alef, madda>, because NFC composes <alef, madda> to <alef-with-madda-above>.

5.2 Shadda

The combining class for shadda (ccc=33) is higher than most vowels; however, it should be displayed closer to the base than the vowels.

5.3 Kasra and kasra-like characters

AMTRA is able to handle the special ligation of kasra and kasra-like characters which are ligated with a shadda or hamza in some styles and appear just below them instead of below the base letter; they still logically follow the shadda or hamza.

5.4 Rationale for exclusion of some marks

Meem above (ccc=230), meem below (ccc=220) and other similar characters are not included in the MCM because their behavior already meets normal expectations. Examples 5a-5c show that the meem is normally kept after fatha, kasra or damma, whereas including meem above and meem below in MCM would have the undesirable effect of moving them in front of fatha, kasra or damma.

Example 5a [Quran1, page 11]


Example 5b [Quran1, page 21]


Example 5c [Quran1, page 19]


Sukun alternate forms

There are three sukun-like marks encoded at U+06DF..U+06E1 that are used in some Quranic orthographies to denote different entities – they may not always represent a sukun. The canonical combining class of these marks is 230, so their ordering in the presence of other combining marks is not affected by AMTRA. However, since the combining class for the sukun is 34, these sukun-like marks will not be treated like a normal sukun in all cases. Users who create data using these alternate sukun characters will have more flexibility than when using the normal sukun. AMTRA does not make them equivalent to U+0652 ARABIC SUKUN, as that would make the algorithm unnecessarily complex and make the usage of CGJ more frequent.


Neither U+0653 ARABIC MADDAH ABOVE (ccc=230) nor U+06E4 ARABIC SMALL HIGH MADDA (ccc=230) are MCM because they are normally displayed above vowel marks.

5.5 Dotted circles

Some rendering engines will insert a dotted circle for what it understands to be an invalid sequence. This is a problem in Arabic script because something that appears invalid may actually be valid text in some lesser known orthography of a minority language or in the Quran. For example, the Microsoft Windows text rendering engine, described in [Microsoft], inserts a dotted circle in combinations of certain Quranic marks that are known to appear with each other in the Quran.

Such spell-checking processes are best implemented at a higher level than a rendering engine. Also, a dotted circle insertion algorithm that displays all canonically equivalent sequences identically is hard to design and the result may be counter-intuitive for its users.

Implementations of the algorithm may be adapted to insert dotted circles by applying the algorithm first and then inserting the dotted circles.

5.6 Other uses for AMTRA

There is no intention or expectation that AMTRA would be applied to stored text. However, there may still be situations unrelated to rendering where AMTRA may be useful, and this UTR does not prohibit such use.

As an example, when a text editor is processing a backspace key, a decision has to be made about what character(s) should be removed from the text. For sequences involving combining marks, if the desire is to remove one mark at a time, users may have an expectation that the outermost marks should be removed first. For Arabic script the AMTRA could be used to identify outermost marks.

5.7 Best combining classes for yet-to-be-encoded combining marks in Arabic

When new combining marks are encoded, 220 should be used for below marks and 230 for above marks. In the special cases where an alternative version of the basic tashkil is encoded, the same combining class as the tashkil could be used, but extreme care should be taken.


[Al-Hilâlî] Muhammad Taqî-ud-Dîn Al-Hilâlî and Muhammad Muhsin Khân (translators) 1417 AH (=1996 CE). The Noble Qur’an: English Translation of the meanings and commentary. King Fahd Complex For The Printing of The Holy Qur’an. ISBN 9960-770-15-X.
[Feedback] Reporting Errors and Requesting Information Online
[Kew] Kew, Jonathan, 2002. Bidi committee consensus on Arabic additions from L2/01-425 http://www.unicode.org/cgi-bin/GetMatchingDocs.pl?L2/02-061 (accessed 1 May 2017).
[Microsoft] Microsoft Typography 2014. Developing OpenType Fonts for Arabic Script. https://docs.microsoft.com/en-us/typography/script-development/arabic (accessed 16 Feb 2018).
[Milo] Milo, Thomas. 2005. Annotations to the printing of the 1924 Azhar Qur'an (U+0670, U+06D6..U+06DB, U+06DD..U+06DF, U+06E0..U+06ED) http://www.unicode.org/cgi-bin/GetMatchingDocs.pl?L2/05-151 (accessed 1 May 2017).
[Pournader] Pournader, Roozbeh. 2009. Proposal to encode four combining Arabic characters for Koranic use. http://www.unicode.org/cgi-bin/GetMatchingDocs.pl?L2/09-419 (accessed 2 May 2017).
[Quran1] Quran example. Al-Baqarah. https://archive.org/stream/quran-pdf/002%20-%20Al-Baqarah (accessed 27 Jul 2017).
[Quran2] Quran example. http://www.dailyayat.com/al-ambiya/21/88 (accessed 27 Jul 2017).
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