The GD&T Guide
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Foundation 01

Datums

The reference system that makes a drawing repeatable in inspection and assembly.

A datum is a theoretically exact reference—such as a plane, axis, or point—used to establish where a part is measured from. The physical feature on the part is the datum feature; the datum is the perfect reference derived from it.

Datum feature vs. datum

A broad mounting face, a bore, or a pin can be a datum feature. During inspection, a fixture, surface plate, mandrel, or CMM setup simulates that physical feature and establishes the ideal datum from it. The two are related, but they are not the same thing.

How a datum reference frame works

When a feature control frame lists A, B, then C, it establishes the setup order. Primary datum A is contacted first, then B, then C. Together they create a stable reference frame so different people can inspect the part the same way.

Choosing useful datums

Good datum features reflect how the part functions, assembles, or is held during manufacturing. A convenient surface is not automatically a useful datum. Start with the features that locate the part in its real application.

Example datum reference frame

ABC

A mounting face may establish datum A, a side face datum B, and a locating bore datum C. A position callout can then locate a hole pattern from that A-B-C setup.

Common mix-up

A datum label placed on a drawing feature does not mean “measure from this edge.” It identifies the feature used to establish the theoretical reference.

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