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

• A. To provide a repeatable reference frame for locating and orienting features during measurement.
• B. To specify surface finish requirements.
• C. To define the color of the part.
• D. To communicate manufacturing cost targets.

Answer: (A)

Datums in GD&T establish a repeatable reference frame that is used to locate and orient features during measurement. They define a coordinate system, typically with primary, secondary, and tertiary datums, so that tolerance zones are applied consistently and parts are gauged the same way each time. This anchoring to a common frame makes measurements and assemblies predictable, reducing variation caused by setup differences. The datum concept is not about surface finish, color, or cost targets, which are controlled by separate specifications.