Perpendicularity is a specific form of Angularity at 90°. ... When you call out Perpendicularity, flatness is implied (you are measuring a surface variation between two parallel planes = Flatness) Perpendicularity is always measured with respect to a datum, where flatness is not.

What datum controls perpendicularity?

Perpendicularity is measured using a height gauge, similar to flatness, however, the gauge (or part) is locked to a 90° datum to measure how perpendicular the surface is. The entire surface has to be measured if it is a flat feature.

How do you inspect perpendicularity?

Hold the square ruler against the target. Measure the gap between the square ruler and the target using a feeler gauge or pin gauge. This gap indicates the perpendicularity.