What is the purpose of a datum in GD&T?

• A. To establish a repeatable reference frame for evaluating tolerances and locating features.
• B. To provide a nominal dimension.
• C. To define material type.
• D. To determine surface finish.

Answer: (A)

Datums in GD&T establish a repeatable reference frame that anchors how tolerances are interpreted and how features are located. By designating specific physical features on the part as datum features, you create a datum reference frame that defines orientation and position for inspection. When a tolerance zone is evaluated, it is measured relative to this frame, ensuring consistent interpretation across parts and inspectors. For example, choosing a flat face as the primary datum sets the main plane, a perpendicular feature as the secondary datum fixes rotation, and a third datum can lock the remaining direction, so hole patterns or features are evaluated in a stable, repeatable way. Without datums, measurements would be ambiguous because there’d be no consistent frame of reference. Nominal dimensions are target sizes, material type isn’t defined by datums, and surface finish is specified separately, so datums aren’t about those attributes.