Which statement best describes a three-datum frame in GD&T?

• A. All three datum features must be oriented to create a reference frame.
• B. Only two datum features are ever required.
• C. A three-datum frame is not used in practice.
• D. Datums do not affect the reference frame.

Answer: (A)

A three-datum frame uses three non-parallel datum features to establish a stable, unique reference in space. These three datums define primary, secondary, and tertiary directions, locking all six degrees of freedom—three translations and three rotations. With three independent datums, the part’s orientation and location are fixed consistently, which is why this frame is widely used in GD&T for both sizing and inspection. If you only had two datums, one rotational or translational degree of freedom would remain unconstrained, leading to ambiguity in the part’s positioning. In practice, three datums are chosen to create a robust, repeatable reference frame.