In typography, a measure of space in relation to an em space. Since the size of an em space is dependent upon the point size, the fractional divisions of an em are also dependent upon the point size. Common relative units are defined as relative-unit spaces per em, and include 18, 36, 54, 72, and 108. In the first case, then, the relative unit consists of 18 units per em, the exact width of which is a function of the point size. See Unit System.