In typography, a concept applied to character width that is no longer universally applicable. All characters of a typeface can be output in a particular point size. The width of the characters increases as the size does; and the widths are programmed to relate to the size. Thus, a 9-point font has widths that are 9-set (width); 18 point is 18-set. Some faces are designed to be somewhat narrower; for example, 9-point on 8.5-set. Some typesetters allow you to change the set size by machine command. Actually, what changes is the space on either side of the character (its total width), but the actual width of the character itself does not change. Thus, 9-point, 8-set actually tightens the character spacing. More appropriately, negative letterspacing commands, in either actual or relative unit values, are used to tighten up spacing (white space reduction).
The concept of set, however, is different as practiced by digitized typesetters and page makeup programs. Here, because characters are made up of dots, one can actually condense the width of a character electronically. A 9-point character can be output at various levels of condensation (or expansion). But here again, the use of the word set is not accurate. The characters are being condensed (or expanded) in programmable increments (12% units or 1% units, for example).