The caliper is sometimes considered a modification of the steel rule by jaws and a vernier scale. The depth gauge is probably even closer to the steel rule than the caliper. This instrument consists of a bar nearly identical to a steel rule and a vernier scale. The vernier scale is slightly modified with a base to rest the depth gauge on the surface of the part being measured. A similar instrument is the depth gauge with no vernier scale attachment. Measurements are then made directly as they would be with a steel rule.
The depth gauge is placed on the part in such a manner so that one or both of the flanges of the vernier base are setting completely flat on the part. The rule, or bar, is then extended down to contact a lower surface. Fine adjustment is accomplished as with a caliper—the instrument is held in place with one hand while the operator clamps the fine adjustment screw and then adjusts the bar up or down so that the vernier head and rule are both contacting their appropriate surfaces. The vernier head is then clamped to maintain correct reading, and the instrument is extracted from position and the measurement read. These instruments are also available with digital electronic readouts.