X-height

The height of a lowercase letter without its ascenders. It decides how large a typeface feels, more than its point size ever does.

Point size measures an invisible box; x-height measures what the eye actually meets, the height of the lowercase body. Two faces at the same nominal size can differ by what feels like a full step, purely on x-height. It is the honest measure of how big type feels.

Practically, x-height decides small-size behaviour. Generous x-heights hold open at captions and interface sizes; elegant low ones collapse. When a face must work at twelve pixels, we compare x-heights before anything else, and adjust nominal sizes so different faces feel level.