The parts of a letter that extend above the X-height (ascender) or below the baseline (descender).
In typography, an ascender is the portion of a minuscule letter in a latin-derivered alphabet that extends above the mean line of a font. That is, the part of a lower-case letter that is taller than font’s X-height.
Ascender, together with descenders, increase the recognizability of words. For this reason, many situations that require high legibility such as road signs avoid using solely capital letters, the all caps style.
Studies made at the start of the construction of the British motorway network concluded that words with mixed-case letters were much easier to read than “all-caps” and a special font was designed for motorway signs. These then became universal across the U.K.
In many fonts intended for body text, such as Bembo and Garamond, ascenders rise the cap height of the capital letters.