Which statement about a datum is correct in GD&T terms?

• A. A datum is a theoretical point with no orientation.
• B. A datum is an optional reference that can be ignored.
• C. A datum is the reference line or plane used to establish the measurement frame.
• D. Datums have no impact on how tolerances are evaluated.

Answer: (C)

In GD&T, the key idea is that a datum provides the fixed reference frame for measuring and evaluating features. A datum isn’t just an abstract point; it is a defined reference surface, line, or axis that establishes the measurement frame—the datum reference frame—from which all tolerances are oriented and located. This frame anchors how distance, position, and orientation are interpreted during inspection, ensuring consistent results across parts.

So the correct statement is that a datum is the reference line or plane used to establish the measurement frame. It captures how the part is supposed to sit in relation to the measurement setup and how tolerances are applied to other features.

Why the other ideas don’t fit: a datum sets orientation (and often location) and isn’t merely a theoretical point with no orientation; it isn’t optional or ignorable in GD&T because tolerances reference the datum frame; and datums do impact how tolerances are evaluated since all measurements are made relative to that defined frame.