Aperture

The opening where a letterform nearly closes, as in a c or an e. Wider apertures read more clearly small and at distance.

Aperture is the doorway of a nearly closed letter: the gap where a c, an e or an s opens to the outside. Wide apertures keep those letters distinct at distance and small sizes; tight ones close into each other, and suddenly c reads as o.

Signage, captions and interface labels live or die on aperture, which is why faces drawn for wayfinding hold theirs wide open. Checking a candidate face, we set the treacherous pairs, c against o, e against a, at the smallest real size. If they hold apart, the face can work.