Computer Type Unit Systems
Each character or space in a font has a set width defined in units. This is a relative measuring system which defines a width relationship between every character and space within a font. The actual width of each unit will vary depending upon the type size selected. For example, a typical font designed on a 1000-unit system may have a capital W defined as 1000-units and a lowercase n as 500-units, or one-half the width of the W. If you select a type size of 1000 point, each unit in the cap W would be one point wide. When the same font is output at 100 point each unit in the W would be one-tenth of a point wide. It is seldom of any concern to the user what unit system a font is based on but it is helpful to understand that a unit system is the method by which a computer keeps track of each character and space typed in order to accurately fill line widths.
QuarkXPress uses a 200-unit system, while Adobe Illustrator and InDesign use a 1000-unit system. It is important to know what unit system your application uses when kerning or tracking. Ten units in QuarkXPress is equal to 50 units in Illustrator or InDesign because their 1000-unit system has five times as many units in a given space than Quark’s 200-unit system. To add more confusion, applications differ on which character they base their unit system on. For example, Quark uses the width of two zeros from a font to determine its 1000-unit base, while InDesign uses the lowercase m.