What is a datum plane?

• A. A curved reference surface.
• B. A machining tolerance.
• C. A dimensionless point.
• D. A flat reference plane established by a datum feature.

Answer: (D)

In GD&T, a datum plane is a flat reference plane used as a baseline for measuring and locating other features, established by a datum feature. This means a physical flat surface on the part is designated as the datum, and that surface defines a precise plane that becomes the reference for the part’s orientation and position in measurements or manufacturing checks. This is why the correct description is a flat reference plane established by a datum feature. A curved reference surface isn’t a datum plane; it would be a curved datum surface. A machining tolerance or a dimensionless point aren’t planes either. For example, if the top face of a block is selected as the primary datum, that flat top defines the datum plane used to orient and locate other features relative to it.