Em (typography)

In typography, an em is a unit of relative width, equal to the current point size: e.g. at 16 points one em is 16 points wide (5.6 mm, using the PostScript point). The em dash (—) and the em space ( ) are each one em wide. An en is half the width of an em.

Traditionally defined as the width of the capital M at the current typeface and point size; in many modern typefaces, however, M is usually less than one em wide.