User Guide

FontLab 4
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S u b s t i t u t i o n Lo o k u p s
Substitution lookups deal with the replacement of the source glyph(s) with
some other glyph(s).
The simplest example of a substitution lookup is the replacement of
lowercase characters by small-caps versions.
Lookups may be context-dependent or context-independent. Context-
independent lookups are applied every time the source sequence of glyphs
is present, like when you want to replace the ‘f’ and ‘l’ sequence with the ‘fl’
ligature. In other cases you may need to apply a substitution only when a
source sequence of glyphs is surrounded by some other glyphs. For
instance you may want to replace an uppercase character with a lowercase
when it is followed by another lowercase character.
The OpenType specification declares the following types of basic
substitutions:
Single Replaces a single glyph with another single glyph: a->A
Ligature
Replaces multiple glyphs with a single glyph: f l -> fl
Multiple
Replaces a single glyph with multiple glyphs: $ -> d o l l a r
Alternate Replaces a single glyph with one of the glyphs in a list: A -> A.version1
or A.version2
All these substitutions may be context independent or context-dependent.
FontLab supports all of the substitutions except the multiple substitutions.