Which statement best defines a datum in GD&T?

• A. A reference point, line, or surface used to establish a datum reference frame for evaluating tolerances.
• B. A basic dimension.
• C. A tolerance modifier.
• D. A projection height.

Answer: (A)

In GD&T, measurements are made from a fixed reference system called the datum reference frame, which is defined by a datum. A datum is a real feature used to establish that frame—its surface, edge, or point becomes the reference element that sets the orientation and location for evaluating tolerances. This is why the statement describing a datum as a reference point, line, or surface used to establish a datum reference frame for evaluating tolerances is the best fit: it captures how datums anchor the measurement system.

A basic dimension is an exact size used to locate features, not the reference system itself. A tolerance modifier is a symbol that changes how much variation is allowed, not the datum. A projection height relates to how far a tolerance zone is projected away from a feature, which is another concept entirely.