GD&T Perpendicularity Definition
In a mechanical part drawing, perpendicularity tolerance allows the designer to specify the degree to which the orientation of a right-angled part feature may vary. The perpendicularity symbol is often used on a drawing to ensure that mating features can be assembled. In most cases, the perpendicularity symbol is applied to a feature-of-size (FOS) with its base dimension. The tolerance zone is created perpendicular to the specified datum, and a part feature, axis, or center plane must lie within it.
In the left figure above, the boxed perpendicularity symbol and tolerance are used to control the center axis of a male feature. The boxed symbols can be read "This axis must lie between two planes perpendicular to the surface at A and spaced 0.1 apart".
The tolerance zone created is indicated by the parallel lines in the rightmost figure, and applies for the entire length of the axis extending from datum plane A. Note that this example only controls the orientation in the plane drawn. The other plane (perpendicular to the above drawing) is unaffected by this orientation control.
When the perpendicularity symbol is applied to a FOS, it is often used in a part or assembly tolerance stack. When the perpendicularity symbol is applied to a surface or line on a part drawing, it is refining the orientation of the surface or line element, acting more like a flatness control for the purposes of performing a tolerance stack.