What is the purpose of a datum in GD&T, and how are primary, secondary, and tertiary datums used to create a datum reference frame?

• A. A datum establishes a theoretically exact reference used to locate features. Primary defines orientation about one axis, secondary defines a second axis, and tertiary defines the third; together they create a datum reference frame for inspection.
• B. A datum is a physical feature used to create tolerance zones.
• C. A datum indicates allowable variation for a feature.
• D. A datum is a symbol used to label dimensions.

Answer: (A)

In GD&T, a datum is an exact reference surface, line, or point used to locate features and define how a part is measured. It comes from a datum feature, a real surface or feature on the part, but the datum itself is treated as an ideal reference for inspection. Three datums—primary, secondary, and tertiary—are used to build a datum reference frame, which provides three mutually perpendicular axes (and an origin) to orient and locate features during measurement.

The primary datum establishes the main orientation, fixing two rotational degrees of freedom and aligning the part relative to one axis. The secondary datum adds a second orientation, constraining the part further and defining the second axis. The tertiary datum completes the frame, fixing the last axis and the remaining translational freedom, so the three axes form a complete reference for inspection.

This frame ensures that tolerances are interpreted consistently with respect to a common, repeatable coordinate system. For example, a flat surface might serve as the primary datum to define the part’s plane, a bore axis as the secondary to constrain rotation about that plane, and a corner or edge as the tertiary to nail down the remaining direction. The datums themselves are ideal references derived from real features, not just tolerance zones or labels.