In typography, the x-height or corpus size refers to the distance between the baseline and the mean line in a typeface. Typically, this is the height of the letter x in the font (the source of the term), as well as the u, v, w, and z. (Curved letters such as a, c, e, m, n, o, r and s tend to exceed the x-height slightly, due to overshoot.) However, in modern typography, the x-height is simply a design characteristic of the font, and while an x is usually exactly one x-height in height, in some more decorative or script designs, this may not always be the case.
Lowercase letters whose height is greater than the x-height either have descenders which extend below the baseline, such as y, g, q, and p, or have ascenders which extend above the x-height, such as l, k, b, and d. The ratio of the x-height to the body height is one of the major characteristics that defines the appearance of a typeface. The height of the capital letters is referred to as Cap height.
In computing, one use of x-height is as a unit of measurement in web pages. In CSS and LaTeX the x-height is called an ex. The use of ex in dimensioning objects, however, is less stable than use of the em across browsers. Internet Explorer, for example, dimensions ex at exactly one half of em, whereas Mozilla Firefox dimensions ex closer to the actual x-height of the font, rounded relative to the font's current pixel height. Thus, the exact ratio of ex to em can also vary by font size within a browser if the determined values are rounded to the nearest whole unit. For example, a browser calculating an x-height of 45% on a font 10 pixels tall may round ex to either 4 pixels or 5 pixels or leave it at 4.5 pixels.