What is the difference between a "datum" and a "datum feature" in GD&T language?

• A. A datum is the actual physical surface used to establish the datum.
• B. A datum is the reference line or plane used to establish the measurement frame.
• C. Datums and datum features are the same.
• D. Datums are only for design, not measurement.

Answer: (B)

In GD&T, you distinguish between the reference concept and the physical feature that creates it. A datum is the reference line, plane, or point that your measurements use as the origin or frame for locating all other features. The datum feature is the actual surface or feature on the part that provides that reference in the real world. So the datum is the theoretical reference, while the datum feature is the physical surface you select to establish that reference. For example, choosing a flat machined surface as a datum feature creates a datum plane that your measurements will align to. The other options blur these roles or misstate their purpose—datums aren’t just the physical surface, and they’re not only for design but for establishing the measurement framework.