Chapter 3 Datums 83 Mutually perpendicular planes Direction of measurements Part 90° 90° 90 ° Example 3-6. The datum reference frame. Example 3-7. The datum feature symbols on the drawing relate to the datum features on the part. Look at this drawing as you refer to Examples 3-8 through 3-10 and the related discussion in the text. secondary, and Datum C is tertiary. The geometric tolerance that controls a feature or features relative to datums must include one or more datum references, as shown with the position tolerance relating the location of the two holes to Datums A, B, and C in Example 3-7. An explanation of the symbols on the drawing in Example 3-7 follows. Example 3-7 follows. Example 3-7