L2/03-041R2

[This document is updated as per the UTC discussion on 8.26; look at the part below the horizontal line.]

I got a question from someone here about the exact definition of "base character". I took a look at it, and the definition is very badly written. We have:

D13 Base character: a character that does not graphically combine with preceding characters, and that is neither a control nor a format character.

• Most Unicode characters are base characters. This sense of graphic combination does not preclude the presentation of base characters from adopting different contextual forms or participating in ligatures.

D14 Combining character: a character that graphically combines with a preceding base character. The combining character is said to apply to that base character.

In determining what D13 actually means in practice, one might start by analyzing it as follows:
- it is a character (so remove Cn, Cs)
- it is not a control or format (so remove Cc, Cf)
- it is not a combining character (so remove Mc, Mn, Me).

But this is not exactly crystal clear. And certainly Zl and Zp (line/paragraph separators) are not explicitly mentioned but must be. The two definitions D13 and D14 are also circular. The definition and notes also do not mention private use characters. We propose the following fix to the text, in light with our new Grand Character Typology in Chapter 2, for the next appropriate version of the Unicode Standard:

D13a Graphic character: a character with the General Categories of Letter (L), Combining Mark (M), Number (N), Punctuation (P), Symbol (S), or Space Separator (Zs).

• Graphic characters specifically exclude the line and paragraph separators (Zl, Zp) and exclude the characters with the General Categories of Other (Cn, Cs, Cc, Cf).
• The interpretation of private use characters (Co) as graphic characters or not is determined by the implementation.
• For more information, see Chapter 2, especially Section 2.4 Code Points and Characters and Table 2-2 Types of Code Points.

D13b Base character: any graphic character except for those with the General Category of Combining Mark (M).

• Most Unicode characters are base characters. In terms of General Category values, a base character is any code point that has one of the categories Letter (L), Number (N), Punctuation (P), Symbol (S), or Space Separator (Zs).
• Base characters are independent graphic characters, but this does not preclude the presentation of base characters from adopting different contextual forms or participating in ligatures.
• The interpretation of private use characters (Co) as base characters or not is determined by the implementation. However, the default interpretation of private use characters should be as base characters, in the absence of other information.

D14 Combining character: a character with the General Category of Combining Mark (M).

• The graphic positioning of a combining character depends on the last preceding base character. The combining character is said to apply to that base character.
• Combining characters consist of all characters with the General Category values of Spacing Combining Mark (Mc), Non-Spacing Mark (Mn), and Enclosing Mark (Me). The interpretation of Private Use characters (Co) as combining
  characters or not is determined by the implementation.
• All characters with non-zero canonical combining class (Cc) are combining characters, but the reverse is not the case: there are combining characters with a zero canonical combining class.