Geometric tolerances specify the maximum variation that is allowed in form or position from true geometry.   The geometric tolerance is, in essence, the width or diameter of tolerance zone within which a surface or axis of hole or cylinder can lie which results in resulting feature being acceptable for proper function and interchangeability.

If a tolerance of form is not specified on a drawing for a feature, then the feature as made will be acceptable regardless of form variation.   The tolerances of form control straightness, flatness, parallelism, angular displacement etc. etc.

The tolerance zone will be one of the following:

• The area within a circle
• The area between two circles
• The area between two equidistant lines or between two parallel straight lines
• The space within a cylinder
• The space between two coaxial cylinders
• The space between two equidistant surfaces or two parallel planes
• The space within a bent pipe

The tolerance frame can be divided into two or more compartments.
These compartments include from left to right

• The symbol for the feature to be toleranced
• The tolerance value..If the tolerance zone is circular or cylinderical it is preceded with a   
• Letters for datums when the toleranced feature is specified in relation to one, or more datums.

If more than one datum is specified then additional partitions are provided

The maximum material condition, when used, is indicated by a symbol placed after the tolerance value, after the datum letter, or both.

See the figures below:

Positioning of Frames /Datum triangles

The features selected for the tolerancing frame or datums triangles are identified as shown below

1. Maximum Material Condition
2. Form Control
3. Flatness
4. Roundness
5. Straightness
6. Squareness/ Perpendicity
7. Form
8. Angularity
9. Position
10. Concentricity/Coaxility
11. Cylindricity
12. Parallelism
13. Runout