14 Geometric Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. GD&T uses a reference coordinate system of datums to locate the features of a part. See Example 1-10. Datums, explained in Chapter 4, are theoretically perfect planes, surfaces, points, lines, axes, or a combination thereof, derived from the true geometric counterpart. The true geometric counterpart refers to the theoretically perfect bound- ary used to establish a datum from a datum feature. A datum feature is the actual feature of the part, such as a surface identified with a datum feature symbol, that is used to establish the datum. A datum feature simulator is a physical boundary used to establish a simulated datum from a specified datum feature. Goodheart-Willcox Publisher Example 1-9. Locating a part using the rectangular coordinate system does not define how the part should be positioned. The plane from which measurements are taken is unspecified, causing confusion and inconsistency when inspecting features.