Display and text faces

The difference is in the details you can afford. At forty pixels and up you can have fine hairlines, tight spacing, dramatic contrast and ornament, because there is room for all of it to resolve. At sixteen pixels in a paragraph, every one of those becomes a liability: the hairlines vanish, the tight spacing closes the words up, and what was elegant becomes grey mush.

Text faces are drawn the other way round — open counters, sturdy strokes, generous spacing, nothing that depends on a detail smaller than a pixel. Set one at ninety pixels and it will not be wrong, it will just be dull, with spacing that looks loose and forms that look plain.

The practical rule is to decide the size before the face, not after. A font that looks magnificent in a specimen is being shown to you at the size it was drawn for, which is rarely the size you are about to use.