What is a "rare" ligature?

If you open the Typography panel from the font panel, you may notice that some fonts have both "Common Ligatures" and "Rare Ligatures". What is that supposed to mean? Is it not up to the user to chose what s/he thinks is rare or common?

There is no strict border between common and rare ligatures. The base assumption is that a common ligature is a ligature that is natural and can be applied without disturbing the text. Take the ligature fi mixing f and i. You can usually apply it whenever an f and and i meet. Most readers will hardly notice, but the text becomes more pleasing to the eye. Common ligatures are usually switched on by default. Common ligature very often start by the letter f. Some good candidates for common ligatures are ff, fl, ffi, fi, ffl, fb, fk and fh.

A "rare" ligature, however, goes out of its way to be noticed. It is usually pleasing to the eye, but at the same time it is so obvious that it disturbes the reading. They should usually not be applied to body text, but they can be applied to headers and titles. The most common "rare" ligature is probably st for s and t. But there are many others. Apple Chancery has rare ligatures for the letter combinations ot, et, sp, ss and es, just to mention a few. Big Caslon and Hoefler Text and many Adobe fonts provide rare ligatures for ct.

Overuse of rare ligatures in Apple Chancery.

The recommendation is to leave the default - common ligatures on, rare ligatures off, unless you find a really good reason to change it.

More information for ligature enthusiasts.