290 Geometric Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. Perpendicularity Tolerance Perpendicularity is the condition of a surface, center plane, or axis at a right angle (90°) to a datum plane or datum axis. A perpendicularity tolerance specifies a toler- ance zone formed by two parallel planes or parallel lines, or a cylindrical tolerance zone, oriented perpendicular (basic 90°) to a datum and within which the controlled surface, axis, or center plane of the feature must lie. Example 7-8 details the perpen- dicularity geometric characteristic symbol and associated feature control frame. Perpendicularity of a Surface When a surface is to be perpendicular to a datum plane, the feature control frame is connected by a leader to the surface, or to an extension line from the surface. See Example 7-9. The actual surface is oriented between two parallel planes at a basic 90° angle to the datum plane. Since the surface is not a feature of size, no material condi- tion modifiers can be used. In Example 7-9, the surface is held perpendicular to one datum plane. It is also possible to hold a surface perpendicular to two datum planes. When done, the sur- face must lie between two parallel planes perpendicular to two datum planes. Both datums are referenced in the feature control frame, as shown in Example 7-10. H = Letter height Symbol Specifications 1.5H 2H H 2H Goodheart-Willcox Publisher Example 7-8. A feature control frame with the perpendicularity geometric characteristic symbol.