Why is a basic dimension critical in a true-position callout?

• A. It influences the size of the tolerance zone.
• B. It defines the measured feature size.
• C. It defines the exact target location; deviation is measured against this target.
• D. It describes the datum features to be used.

Answer: (C)

The essential idea is that a true-position callout measures how far the actual feature is from an exact target location. The basic dimension provides that exact target position (the ideal location relative to the datum references), and there is no tolerance on the basic dimension itself. The positional tolerance in the feature control frame then limits how far the actual feature’s position can deviate from that fixed target. Without a basic dimension to define the precise target, there would be no reference point to compare the true position against.

So the basic dimension is critical because it fixes the exact target location, and the deviation is measured against this target.