Model-Based Definition > Model-Based Definition > Geometric Dimensioning and Tolerancing Advisor > Advisor Message Help > Feature Messages > Form Is Not Fully Constrained
Form Is Not Fully Constrained
Not all aspects of form have been defined by the tolerances that have been applied to the feature or feature of size. The form of a feature or feature of size may be constrained in one of the following ways:
• A form geometrical tolerances (flatness, straightness, roundness, or cylindricity) applied to the surface of the feature.
• A non-form geometrical tolerance applied to the surface of the feature (e.g., perpendicularity applied to a planar surface imposes an equal implied flatness control to that surface).
• A size dimension with the envelope requirement applied to a feature of size (i.e., form variation must be less than or equal to the size tolerance).
• General geometrical tolerances, as specified by a reference to ISO 2768.
It is generally considered to be good practice to specify the limits of form for each surface of the model (either explicitly or implicitly).