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The CID-keyed font (also known as CID font, CID-based font, short for Character Identifier font) is a font structure, originally developed for PostScript font formats, designed to address a large number of glyphs. It was developed to support ideographic East Asian character sets, as these comprise many more characters than the Latin, Greek and Cyrillic writing systems.
Adobe developed CID-keyed font formats to solve problems with the OCF/Type 0 format, for addressing complex Asian-language (CJK) encoding and very large character sets. CID-keyed internals can be used with the Type 1 font format for standard CID-keyed fonts, or Type 2 for CID-keyed OpenType fonts. CID-keyed fonts often reference "character collections," static glyph sets defined for different language coverage purposes. Although in principle any font maker may define character collections, Adobe's are the only ones in wide usage. Each character collection has an encoding which maps Character IDs to glyphs. Each member glyph in a character collection is identified by a unique character identifier (CID). Such CIDs are generally supplemental to other encodings or mappings such as Unicode.
Character collections are uniquely named by registry, ordering and supplement, such as "Adobe-Japan1-6." The registry is the developer (such as Adobe). The so-called "ordering" gives the purpose of the collection (for example, "Japan1"). The supplement number (such as 6) indicates incremental additions: for a given language, there may be multiple character collections of increasing size, each a superset of the last, using a higher supplement number. The Adobe-Japan1-0 collection is 8284 glyphs, while Adobe-Japan1-6 is 23,058 glyphs.
CID-keyed fonts may be made without reference to a character collection by using an "identity" encoding, such as Identity-H (for horizontal writing) or Identity-V (for vertical). Such fonts may each have a unique character set, and in such cases the CID number of a glyph is not informative; generally the Unicode encoding is used instead, potentially with supplemental information.
CID-keyed fonts internally have their character sets divided into "rows," with the advantage that each row may have different global hinting parameters applied.
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