84 Geometric Dimensioning and Tolerancing The part is then positioned against the secondary datum, as shown in Example 3-9. With the part held against the primary and secondary datums, dimensions can be verified from the secondary datum inspection table surface to features that are dimensioned from the secondary datum. Primary datum plane Example 3-8. The surface of the part labeled as the primary datum is placed on the surface of the inspection table. Secondary datum plane 90° Example 3-9. The part is now positioned against the secondary datum. The surface of the part labeled as the primary datum is placed on the surface of an inspection table or manufacturing inspection equipment, as shown in Example 3-8. Now measurements can be made from the primary datum inspection table surface to features that are dimensioned from the primary datum.