This information provided by Bryan R. Fischer, of TDP360 LLC.
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Form, Size, Orientation, Location are the geometric characteristics used to describe part geometry.
For individual features Form and Size We can use GD&T to control the form (shape) of every feature Every feature has a form (shape). Only features with opposed points have size. Relationships between features Orientation and Location We can use GD&T to unambiguously control the orientation and location of features relative to each other and to a datum reference frame. The ability to do this is unique to GD&T and it is one of the most important benefits of GD&T. |
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Purpose of Form Tolerances
Form tolerances control the form (shape) of individual (singular) features. Form tolerances do not and cannot control the relationships between features. Thus, form tolerances cannot be related to a datum reference frame. For example, the datum features referenced in 
and 
are not allowed.
Except when applied to the main primary datum feature on a part, usually, form tolerances are applied to refine (restrict) the variation allowed by other tolerances. Thus, form tolerance values are often smaller than other tolerances. This is not always the case, but it is common.
Form Tolerances
Form tolerances are geometric tolerances. There are four form tolerances in GD&T.
| Circularity may only be applied to nominally-round geometry. Cylindricity may only be applied to nominally-cylindrical geometry. Flatness may only be applied to nominally-flat geometry. Straightness may only be applied to geometry that includes nominally-straight lines, such as a flat plane or along the length of a cylinder or cone. |
Circularity may only be applied to nominally-round geometry.
Cylindricity may only be applied to nominally-cylindrical geometry.
Flatness may only be applied to nominally-flat geometry.
Straightness may only be applied to geometry that includes nominally-straight lines, such as a flat plane or along the length of a cylinder or cone.
Reminder
In GD&T, nominal refers to perfect geometry. So nominally-flat means the surface is defined as perfectly flat in a CAD model or on a drawing.
Reminder
Rule #1 (The Envelope Principle) only applies to regular features of size. Rule #1 requires regular features of size to have perfect form if produced at MMC. The directly-toleranced dimension that defines the regular feature of size controls the feature’s form and size.