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It is essential to be able to identify a feature of size when reading geometric tolerances. Many geometric tolerances, where applied to a feature of size, are controlling the feature’s center point, axis or center plane. If you do not recognize the feature as a feature of size, you might think the tolerance is being applied to the surface.
Features of size are unique because they:
Caliper Rule -- Opposed Points This rule is not found in the Standard but it is pretty helpful. The left end of the caliper shown is used if the feature being measured has opposed points. Since these features have a toleranced dimension and contain opposed points, they are features of size. Toleranced dimensions that are measured using the probe on the other end of the caliper do not have opposed points and are not features of size. This rule of thumb is intended to help you identify features of size and is not intended to suggest that calipers are the best way to measure size dimensions.