Usually the arm to which the skid is tied pivots a long distance away from the measurement. The skid assembly and transducer are designed to measure the difference in height between the skid height and the stylus tip height. This approach is therefore suitable for roughness profile measurement only. In the simplest skidded profiling instruments the stylus rides on the surface and measures height relative to a skid which also rides on the surface. The skid can be in line with the diamond or beside it as the assembly traverses the part.
Several alternatives are in use for the geometry of the skid relative to the stylus tip.
A single skid can ride in front of, behind, or in line with the diamond. More commonly two skids are used that ride on either side of the diamond. A final alternative is a single skid with the diamond tip protruding down from its center. For some applications, for example measuring round parts, it may be desirable to use two skids to establish the reference height, eliminating the pivot from the measurement. Two skids are occasionally used as the reference for round or unusually shaped parts.