Which statement about a basic dimension is accurate?

• A. It is an exact size or location used to locate features within a true position tolerance and carries no tolerance.
• B. It requires a tolerance.
• C. It defines the datum reference frame.
• D. It specifies a projection height.

Answer: (A)

A basic dimension is an exact value used to define the true location or size of a feature, and it carries no tolerance of its own. This precision provides the ideal target—true position, for example—from which the allowable variation is defined by a geometric tolerance such as true position and the datum reference frame. In other words, the actual feature must fall within the tolerance zone set by the geometric tolerance relative to the datums, not be constrained by any tolerance on the basic dimension itself.

Why this is the best fit: basic dimensions are intentionally tolerance-free because they establish the exact targets used by the geometric tolerances. The datum reference frame is set by the datums themselves (datum features and their order), not by a basic dimension. Projection height is unrelated to basic dimensions; it’s governed by other GD&T symbols and controls.