In GD&T, which statement best describes datums?

• A. They are used to define the primary reference frame
• B. They are temporary marks drawn on the part
• C. They are established reference surfaces or features used to define a measurement frame
• D. They are only used for color coding

Answer: (C)

In GD&T, datums are established reference surfaces or features that define the measurement frame used to locate and orient all other features on the part. They create a datum reference frame by selecting a primary datum surface (or plane), followed by secondary and possibly tertiary datums, which together fix the part’s position and orientation for tolerance application. These datum features can be actual surfaces, edges, or recognized features of size that are used consistently during manufacturing and inspection. They are not temporary marks or color codes; they are deliberate, repeatable references that ensure geometrical tolerances are interpreted the same way each time the part is measured.