Chapter 3 Datums 87 The secondary datum plane constrains two degrees of freedom: • One translational in y. • One rotational in w. The tertiary datum plane constrains one degree of freedom: • One translational in x. Multiple Datum Reference Frames Depending on the functional requirements of a part, more than one datum reference frame can be established. In Example 3-12, datums X, Y, and Z constitute one datum reference frame, while datums L, M, and Y establish a second reference frame. The relationship between the two datum reference frames is controlled by the angularity tolerance on datum feature L. Datum M is the axis of the large hole to which the datum feature symbol is connected. Datum axes are discussed later in this chapter. For parts with inclined datum features, as in Example 3-12, the datum feature simulator plane is oriented at the basic angle of the datum feature. The related datum reference frame plane passes through the vertex of the basic angle and is mutually perpendicular to the other datum planes. Represents datum axis Example 3-12. Multiple datum reference frames.