3. The Glyph Substitution Table (GSUB)

The Glyph Substitution table (GSUB) contains information for substituting glyphs to render the scripts and language systems supported in a font. Many language systems require glyph substitutes. For example, in the Arabic script, the glyph shape that depicts a particular character varies according to its position in a word or text string (see Figure 3a). In other language systems, glyph substitutes are aesthetic options for the user, such as the use of ligature glyphs in the English language (see Figure 3b).

Figure 3a. Isolated, initial, medial, and final forms of the Arabic character HAH

Figure 3b. Two Latin glyphs and their associated ligature