What is a skid?

A skid is a part of the gauge that has a radius large enough to prevent movement in and out the roughness characteristics of the surface. The stylus and the skid are usually independent in their height (Z) movement but move together in the measurement direction. Surface deviations are recorded as the difference between the stylus and the skid movement in the Z direction. In other words, the skid acts as the straightness datum ? it ?skids? over the top of the surface