Measure the perpendicularity (SQR)

Two straight lines meet at an angle of exactly 90° with the right angle.
The right angle condition specifies the accuracy of how vertical a measurement object is to the reference plane or reference line. The tolerated elements are straights or planes.

The tolerance zone is limited by two parallel straights or planes at distance t, which are vertical to the reference.

Technical drawing: Measuring perpendicularity against a plane
Technical drawing: Measuring perpendicularity against the axis