In typography, a series of relative width values assigned to characters as a means of describing the relation between their widths.
When Tolbert Lanston invented the Monotype, he wanted to separate the functions of input and output, and he needed a method that would let the operator know when to end a line for justification. Arithmetic was the best idea: he would add up character widths. To store the widths of every character in every point size of every typeface would have been prohibitive, so he created relative widths. In any typeface, all characters are proportional to one another, and from point size to point size that proportionality remains the same. Thus, if the width of every character is described as a multiple of some value, then those numerical relationships will still be valid, no matter what the size is.
For example, an "a" in 9 points might be eight-eighteenths; the "a" in 72 points would also be eight-eighteenths. The real width that these characters occupy is differentiated by multiplying those relative values by the point size:
9-point x 8-unit "a" = 72
72-point x 8-unit "a" = 576
Thus, one set of values serves for all sizes of a particular typeface. Lanston's base was 18, which served phototypesetting for many years. In order to speed up film-font manufacture, suppliers moved to 36-, 54-, and 72-unit systems. Also known as the unit count system.
(See also Unitized Font.)