X-height

It is measured on the x because the x has no curve to overshoot the line and no ascender, so it is the honest number. Everything else follows from it: how large a face looks at a given point size, how much of the line the lowercase fills, and how much room is left for the ascenders and descenders that carry a word’s shape.

A large x-height makes a face look bigger than its point size says, which is why two fonts set at 16px can look a size apart. That is usually an advantage on screen — more of the letter is in the pixels you have — but it costs you word shape, because the ascenders and descenders get short and every word starts to look like a rectangle.

This is the single most useful thing to compare when two faces are shortlisted, and it is invisible in a specimen that shows them at different sizes. Set them in the same sentence at the same size and the difference is obvious immediately.