296 Geometric Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. Alternately, the desired effect works if a slot runs the length of the ∅25 cylinder, at the circumference, and is identified as datum feature B. Then, adding the cylindri- cal tolerance zone in the feature control frame and including datum B as the second- ary datum establishes the resistance needed to constrain the degrees of freedom. See Example 7-13. The ∅25 cylinder resists movement in the Y and Z directions and resists rotation about the Y and Z axis, and there is resistance about the X axis when looking into the end of the cylinder. Another application that can require a perpendicularity tolerance specification is a cylindrical feature such as a pin or stud. In this situation, the feature axis is within a cylindrical tolerance zone perpendicular to a datum plane. The feature control frame is associated with the diameter dimension and a diameter symbol precedes the geo- metric tolerance to specify a cylindrical tolerance zone, as shown in Example 7-14. Regardless of feature size is default as shown in Example 7-14A. Example 7-14B shows using the maximum material condition modifier and the allowable tolerance values at possible feature sizes. The feature axis must also be within the specified positional tolerance. Perpendicularity of a Center Plane A width feature, such as a slot, can be specified as perpendicular to a datum plane. In this application, the feature center plane is held within two parallel planes perpen- dicular to a datum plane. When a positional tolerance is applied, the center plane must also be within the specified positional tolerance. See Example 7-15.