An ascender is the portion of a minuscule letter in a Latin-derived alphabet that extends above the mean line of a font. That is, the part of a lower-case letter that is taller than the font's x-height.
Ascenders, together with descenders, increase the recognizability of words. For this reason, British road signs no longer use all capital letters.
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. See Road signs in the United Kingdom.