In a geometric dimensioning and tolerancing system, datum references primarily serve to define what?

• A. The material properties
• B. The surface finish tolerances
• C. The coordinate system used to locate features
• D. The allowable manufacturing tolerances

Answer: (C)

In GD&T, datum references establish a datum reference frame that defines the coordinate system used to locate features on a part. By tying surfaces, edges, or features to fixed reference planes or axes, they create a consistent frame for measuring and inspecting geometry. This frame is what other tolerances refer to when specifying how features must relate to one another—for example, how a hole’s axis must be positioned relative to the datums.

Datum references themselves aren’t about material properties, surface finish, or how large or small a feature tolerances must be. Material properties come from material specifications, surface finish from roughness symbols, and actual tolerance values come from the feature tolerances and control frames. The datums simply provide the spatial reference used to interpret and apply those tolerances consistently.