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FormFormAll form tolerances apply to single, or individual, features; consequently, form tolerances are independent of all other features. No datums apply to form tolerances. The form of individual features is automatically controlled by the size tolerance—Rule #1. When the size tolerance does not adequately control the form of a feature, a form tolerance may be specified as a refinement. Except for straightness of a median line and of a median plane, all form tolerances are surface controls and are attached to feature surfaces with a leader or, in some cases, an extension line. No cylindrical tolerance zones or material conditions are appropriate for surface controls.…
Gene R. Cogorno: Geometric Dimensioning and Tolerancing for Mechanical Design. Form, Chapter (McGraw-Hill Professional, 2006), AccessEngineering