Most geometric tolerances reference one or more datums. A datum reference consists of a datum identifier that refers to one or more datum features. A datum identifier is usually a single datum letter such as A or D, but it can be a sequence of a few letters. In PC-DMIS, a datum identifier can refer to any of these:
A single feature, such as a plane, line, point, cylinder, circle, cone, sphere, or width—this is often called a single datum.
A pattern of like features of size that all have the same nominal size and size tolerance; this case is restricted to cylinders, circles, spheres, and widths—this is often called a datum pattern.
A geometric tolerance may reference two or more datum identifiers at once by joining them with a hyphen, such as C-D; this is often called a common datum. In PC-DMIS, common datums are restricted to these features:
Coaxial cylinders and/or circles that have different nominal sizes and/or size tolerances
Offset parallel planes
More: