In GD&T, what is the purpose of datums?

• A. Datums provide a reference frame for locating features.
• B. Datums specify color.
• C. Datums indicate tolerances.
• D. Datums define material type.

Answer: (A)

Datums provide a fixed reference frame for locating and measuring features on a part. In GD&T, a datum is a precisely defined plane, axis, or point that establishes how the part’s geometry is measured and inspected. By designating primary, secondary, and tertiary datums, you create a datum reference frame that fixes three translational and three rotational degrees of freedom. When a feature tolerance is specified relative to these datums, every measurement is made with respect to the same frame, ensuring consistent interpretation between manufacturing and inspection. A datum is not about color, material, or tolerances themselves; it is about where the measurements refer to. For example, using a flat machined surface as the primary datum defines the reference plane; another feature’s location or orientation is then controlled relative to that plane, so the hole’s axis or the edge’s position is measured consistently.