What is the role of a datum axis in GD&T?

• A. It defines the straightness of a feature.
• B. It fixes the location in all three axes.
• C. It controls material properties.
• D. It provides the rotational reference for circular features and runout tolerances.

Answer: (D)

A datum axis in GD&T provides the rotational reference line for circular or cylindrical features, and it defines how runout tolerances are applied and measured. By establishing this axis, you fix the orientation of those features with respect to the datum frame, which is essential when evaluating how a surface deviates as the part is rotated about that axis. It’s not about straightness, nor about fixing the part’s location in all three directions, nor about material properties. So the datum axis best serves as the rotational reference for circular features and runout tolerances.