112 Geometric Dimensioning and Tolerancing Be sure you notice the difference between the datum feature symbol associated with the datum axis discussed earlier and the datum center plane. The drawings look similar. The datum axis is on a cylindrical feature such as an external shaft or an internal hole. Look carefully at the drawing views and notice that the cylindrical datum feature is dimensioned with a diameter (refer to Example 3-29). The datum center plane is the plane that splits a symmetrical feature such as a slot or tab. The dimension associated with the center plane datum does not have a diameter symbol because the feature is not round. The datum feature symbol is placed in one of the following ways. • The symbol is centered on the opposite side of the dimension line arrowhead. • The symbol replaces the dimension line and arrowhead when the dimension line is placed outside of the extension lines. • The symbol is placed on a leader shoulder. • The symbol is placed below and attached to the center of a feature control frame. Simulated Datum Center Plane The simulated datum center plane is the center plane of a perfect rectangular inspection device that contacts the datum feature surface. For an external datum feature, the datum center plane is established by two parallel planes at minimum separation, as shown in Example 3-40. For an internal datum feature, the datum center plane is established by two parallel planes at maximum separation, as shown in Example 3-41. This application is valid only when datum features are specified at RMB. RMB is described in Chapter 4. Datum center plane A Datum feature A Datum feature simulator of datum feature A Parallel planes at minimum separation (RMB) Datum feature simulator Part Datum feature simulator Example 3-40. Simulated datum center plane for an external datum feature only when a datum feature is specifi ed at RMB.