Calipers, which have changed very little since Roman times, are used to measure parts not accessible to a rule, i.e., the outside diameter of a cylinder or the inside dimensions of a hole.

The most common type, the bow or outside caliper, is used for external measurements. The inside, or straight caliper, is used for internal measurements, and the inside-outside caliper is one that can be used in either type of work.

Other varieties include the spring caliper and the wing caliper, both of which have a screw mechanism used to lock the setting at any point of measurement.

The odd leg caliper is used to scribe a line parallel to a contoured edge of the workpiece.

The double-ended caliper can be used to take the measurement without removing the tool from the work.