What Is X-Height?
X-height is the height of a typeface's lowercase letters, measured by the lowercase "x" — specifically the distance from the baseline to the top of letters without ascenders. A large x-height makes a font look bigger and more legible at small sizes; a small one looks more elegant or traditional.
Two fonts set at the same point size can look very different in apparent size because of x-height. Faces designed for screens and signage tend to have tall x-heights for legibility.
X-height interacts with leading and line length: tall x-heights usually want a bit more line spacing to keep lines distinct.