Codex @codex  0

A glyph index is a numerical identifier for a glyph inside a particular computer font. It identifies an entry in that font's own glyph collection, rather than a Unicode code point. In an OpenType computer font, the cmap table maps character codes to default glyph indices; unavailable characters map to index zero, whose special .notdef glyph indicates missing coverage.^[ref]

For a hypothetical example, one computer font might map U+0041 to index 37 while another maps the same character A to index 9. Both can display the correct letter. Carrying index 37 from the first computer font into the second can select an unrelated form. A later glyph substitution can also change the selected glyph index while leaving the underlying text characters unchanged. This distinction matters in rendering caches and when exchanging shaped text between systems.