calt — Contextual Alternates
calt (Contextual Alternates) replaces default glyphs with alternate forms that join better, in specified situations.
The specification’s example is Caflisch Script, a cursive typeface, where o is replaced by o.alt2 when followed by an ascending letterform. It is used in scripts designed to have glyphs join.
The programming fonts Fira Code and JetBrains Mono form symbol ligatures such as => and != with calt alone, not liga or dlig (verified: neither font has a liga or dlig feature at all, and calt is implemented with Lookup Type 6). The specification does not state why calt is chosen for this.
Example
Section titled “Example”M is replaced only when immediately followed by N.
Without calt
MN MM
With calt
MN MM
.fea source
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Writing it in a feature file
Section titled “Writing it in a feature file”The rules are written with Lookup Type 6.
Without rules
MN MM
With rulesMN MM
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