rlig — Required Ligatures
rlig (Required Ligatures) gathers ligatures required for the correct display of a script.
In Arabic, ل (lam) followed by ا (alef) always takes the single form لا. This is not a matter of taste but a form the script requires. The same happens in the Syriac script.
Example
Section titled “Example”This font has no Arabic glyphs, so the example uses a rule that forms a ligature from a doubled Cyrillic ї (U+0457).
.fea source
Choosing among the ligature features
Section titled “Choosing among the ligature features”Five features form ligatures, rlig among them.
| Feature | Purpose |
|---|---|
rlig |
Ligatures required for correct display of a script. Always applied |
liga |
Ligatures in common use. On by default |
clig |
Ligatures recommended only in a specific context. On by default |
dlig |
Ligatures used for special effect, at the user’s preference. Off by default |
hlig |
Ligatures once in common use that now appear anachronistic. Off by default |
The specification states that control of rlig should not generally be exposed to the user. It applies whenever the script that needs it is in use, without any judgment call by the maker or the user.
liga, which also forms ligatures, applies ligatures in common spelling at the maker’s discretion. The ffl ligature, for example, is not required by the script — it is simply one the maker judges should be used in normal conditions. Whether the script requires the form, or it is a matter of taste, is what separates the two.
Writing it in a feature file
Section titled “Writing it in a feature file”The rules are written with Lookup Type 4.