Understanding Datums

A clear reference is necessary for specifying a target’s orientation tolerance, location tolerance, and profile tolerance—that clear reference is the datum. A datum is a theoretical, ideal feature such as plane, line, or point, which does not actually exist: this makes it difficult to understand. Nevertheless, the understanding of datums is a necessity to understanding GD&T.

This section introduces the characteristics of datums and the basics of common datums and datum systems, both of which are made up of multiple datums.