X height

In typography, the x-height or corpus size is the distance between the baseline and the mean line of lower-case letters in a typeface. Typically, this is the height of the letter x in the font (the source of the term), as well as the v, w, and z. (Curved letters such as a, c, e, m, n, o, r, s, and u tend to exceed the x-height slightly, due to overshoot.) One of the most important dimensions of a font, x-height is used to define how high lower-case letters are compared to upper-case letters.

Display typefaces intended to be used at large sizes, such as on signs and posters, vary in x-height. Many have high x-heights to be read clearly from a distance. This, though, is not universally the case: some display typefaces such as Cochin intended for publicity uses have low x-heights, to give them a more elegant, delicate appearance. Many sans-serif designs that are intended for display text have high x-heights, such as Helvetica or, more extremely, Impact.

Medium x-heights are found on fonts intended for body text, allowing more balance and contrast between upper- and lower-case letters and a brighter page. They then increase again for optical sizes of font designed for small print, such as captions, so that they can be clearly read printed small.

High x-heights were particularly common in designs in the 1960s and 70s, when International Typeface Corporation released variations of older designs with boosted x-heights; notable examples of this trend include Avant Garde Gothic and ITC Garamond. More recently, some typefaces such as Mrs Eaves and Brandon Grotesque have been issued with distinctively low x-heights to try to create a more elegant appearance. While computers allow fonts to be printed at any size, professional font designers such as Adobe issue fonts in a range of optical sizes optimized to be printed at different sizes. As an example of this, Mrs Eaves exists in two versions: an original style intended to give an elegant, bright appearance, and a less distinctive 'XL' design intended for body text.

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Wikipedia