In typography, the x-height, or corpus size, is the distance between the baseline and the mean line of lowercase 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 letters 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; i has a dot that tends to go above x-height.) One of the most important dimensions of a font, x-height defines how high lowercase letters without ascenders are compared to the cap height of uppercase 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 universal: some display typefaces such as Cochin and Koch-Antiqua intended for publicity uses have low x-heights, to give them a more elegant, delicate appearance, a mannerism that was particularly common in the early twentieth century. Many sans-serif designs that are intended for display text have high x-heights, such as Helvetica or, more extremely, Impact.