What is the purpose of a 'box' around a dimension on a GD&T drawing?

• A. A basic dimension, exact and not directly toleranced.
• B. A dimension that requires a tolerance band.
• C. A dimension that is a derived value.
• D. A reference dimension.

Answer: (A)

Boxed dimensions are basic dimensions: exact, theoretically perfect values that define the true geometric relationship between features. They are not given tolerances themselves; the allowable variation comes from the geometric tolerance specified in the feature control frame and tied to datums.

This is why the boxed number is so important—it sets the precise reference for locating, aligning, or sizing features, while the actual permissible deviation is controlled by the tolerance zone (not by that boxed value). For example, the boxed distances used to locate a hole pattern establish the intended true position; the hole’s position tolerance is then applied in its feature control frame relative to datums, not as a tolerance on the boxed distance itself.

Other options don’t fit because a boxed dimension isn’t a tol­erance band itself, and it isn’t a mere reference value shown in parentheses. It’s the exact, un-toleranced reference that the GD&T tolerance zones use to constrain the feature.