AutoCAD 2008 User's
Guide > Annotate
Drawings > Dimensions and
Tolerances > Add Geometric
Tolerances >
Datum Reference Frames
The tolerance values in the feature control frame are followed by up to three optional datum reference letters and their modifying symbols. A datum is a theoretically exact point, axis, or plane from which you make measurements and verify dimensions. Usually, two or three mutually perpendicular planes perform this task best. These are jointly called the datum reference frame.
The following illustration shows a datum reference frame verifying the dimensions of the part.
