Chapter 1 Geometric Dimensioning and Tolerancing Essentials 15 Copyright Goodheart-Willcox Co., Inc. In Example 1-10, notice datum feature symbols A, B, and C and reference to those symbols in the feature control frame. A feature control frame (FCF) is a rectangular symbol used to define the geometric tolerancing characteristics of a feature. The fea- ture control frame is divided into compartments with the geometric characteristic symbol in the first compartment, followed by the tolerance value and any modifiers in the second compartment, and applicable datum feature references in the third com- partment. The datum feature references in the feature control frame in Example 1-10 tell you that datum feature A is primary, datum feature B is secondary, and datum feature C is tertiary. Example 1-11 shows the face plate constrained in a set of three mutually per- pendicular intersecting datum planes and three mutually perpendicular axes at the intersection of those planes. An inspector places datum feature A on the surface of an inspection table or manufacturing inspection equipment first to take measurements from simulated datum A to features dimensioned from datum feature A. A simulated datum is a point, axis, line, or plane, or a combination thereof, derived from a datum feature simulator. The part is then positioned against datum feature simulator B, allowing dimensions to be verified from simulated datum B to features dimensioned from datum feature B. Finally, the part touches datum feature simulator C and is fully confined, allowing all measurements to be made from the simulated datum planes to related features on the part. Datum feature symbol Feature control frame Primary datum feature (A) Secondary datum feature (B) Tertiary datum feature (C) Datum feature reference order constrain datum feature A first, B second, and C third Two diameter 14±0.2 holes Goodheart-Willcox Publisher Example 1-10. In the GD&T system, a reference coordinate system of datums specifies how a part is positioned when taking measurements. Only the datum feature symbols, hole size dimension, and feature control frame associated with the hole size dimension are shown for simplicity.