When interpreting datum reference frames that contains both axis (or center-plane) datums and planner datums, the datum precedence plays a major role in the final part tolerances. In the top portion of Figure 16 the datum portion of the feature control frame is left blank. In the bottom portion of the Figure 16 the datum reference frames are completed in three different methods of datum precedence. Panel A: 'A' is primary RFS and 'B' is secondary datum. A movable gage will be required which will establish the attitude of the part, and feature B may have linear contact with datum plane B. Datum plane B must be perpendicular to the axis of the gage. Panel B: First the datum plane B will be established by three point contact with the planner surface feature B. Then a movable gage will be closed on to the feature B with gage axis perpendicular to A. The axis of the gage when it is touching the part will establish the datum axis B. Using similar logic, the application at Panel C is self-explanatory.

Figure 16. A datum precedence example (Top)