The thickness of metal may be determined by means of a thickness gauge, which consists of many levers, varying in thickness by thousandths of an inch, and each designated by a number. By applying these levers successively, it is possible to determine the thickness of a piece of metal. A similar arrangement of plugs, called thickness plugs, are used for determining the diameter of a hole.

Fig. 201. - Vernier.

Fig. 202. - Vernier Caliper. For determining accurately the depth of gear teeth.