80 Geometric Dimensioning and Tolerancing A datum feature simulator can be one of the following: • Maximum material boundary (MMB). • Least material boundary (LMB). • The actual mating envelope. • A tangent plane. • A mathematically defined contour. Datum feature simulators have the following requirements: • Perfect form. • Basic orientation to each other for all the datum references in the feature control frame. • Basic location relative to other datum feature simulators for all the datum references in the feature control frame, unless a translation modifier or movable datum target symbol is specified. • Movable location when a translation modifier or movable datum target symbol is specified. • Fixed in size when MMB or LMB is specified. • Adjustable in size when RMB is specified. In actual practice, measurements cannot be made from theoretical datum features or datum feature simulators. This is why manufacturing inspection equipment is of the highest quality for making measurements and verifying dimensions even though they are not perfect. Datum plane: The theoretically exact plane established by the simulated datum of the datum feature. Simulated datum: A point, axis, line, or plane consistent with or resulting from processing or inspection equipment, such as a surface plate, inspection table, gage surface, or a mandrel. The simulated datum plane in Example 3-2 is the plane derived from the physical datum feature simulator and coincides with the datum plane when the datum plane is in contact with the simulated datum plane. Surface of manufacturing or verification equipment Datum plane Theoretical datum feature simulator in contact with simulated datum plane Part Physical datum feature simulator Simulated datum plane Plane derived from the physical datum feature simulator Datum feature Example 3-2. Datum plane, datum feature, and the simulated datum plane. Datums are to be treated as if they are perfect even though they may not be perfect.