In skidded gages, the sensitive, diamond-tipped stylus is contained within a probe, which has a skid that rests on the workpiece. Skidded gages (upper) measure roughness only. Skidless gages (lower) use an internal precision surface as a reference. This enables skidless gages to be used for measurements of waviness and form, in addition to roughness. Source: Mahr Federal Inc.