Universal Character Set - Differences Between ISO 10646 and Unicode

Differences Between ISO 10646 and Unicode

ISO 10646 and Unicode have an identical repertoire and numbers — the same characters with the same numbers exist on both standards, although Unicode releases new versions and adds new characters more often. The difference between them is that Unicode adds rules and specifications that are outside the scope of ISO 10646. ISO 10646 is a simple character map, an extension of previous standards like ISO 8859. In contrast, Unicode adds rules for collation, normalization of forms, and the bidirectional algorithm for right-to-left scripts Hebrew and Arabic. For interoperability between platforms, especially if bidirectional scripts are used, it is not enough to support ISO 10646; Unicode must be implemented.

To support these rules and algorithms, Unicode adds many properties to each character in the set such as properties determining a character’s default bidirectional class and properties to determine how the character combines with other characters. If the character represents a numeric value such as the European number ‘8’, or the vulgar fraction ‘¼’, that numeric value is also added as a property of the character. Unicode intends these properties to support interoperable text handling with a mixture of languages.

Some applications support ISO 10646 characters but do not fully support Unicode. One such application, Xterm, can properly display all ISO 10646 characters that have a one-to-one character-to-glyph mapping and a single directionality. It can handle some combining marks by simple overstriking methods, but cannot display Hebrew (bidirectional), Devanagari (one character to many glyphs) or Arabic (both features). Most GUI applications use standard OS text drawing routines which handle such scripts, although the applications themselves still do not always handle them correctly.