Form tolerances straightness flatness circularity and cylind

Form tolerances (straightness, flatness, circularity, and cylindricity) do not require datum references. We do not put datum references in feature control frames that control form tolerances. Why is this? Why do form tolerances not require any references to a datum?

Solution

Datums are required when their is a need of taking reference from a specified point,axis or plane. Form tolerances are defined in such a way that they do not require any datum. For example flatness is defined as the volume between two parallel planes in which the surface should lie to be called as flat.Similarly for straightness , flatness and cylindricity so they do not require any datum.