The principle parts of a vernier caliper are outside jaws, inside jaws, main scale, vernier scale, depth probe, and lock screw. However, depending on the manufacturer, the depth probe might not be attached with some vernier calipers.
A vernier caliper can directly measure the distance between two ends with high precision. In 1631, French mathematician Pierre Vernier invented the modern "vernier scale." A vernier scale is constructed with fixed and equally spaced out marks, which are a specific fraction of the unit that the main scale is using. A vernier caliper makes use of this scale to enable the user to measure distances more accurately than with ordinary scales. When measuring with a vernier caliper, the user must first find the main scale reading. If the reading lies between two main scale marks, then the smaller value is taken as the main scale reading. Next, the user has to find out which vernier scale mark is aligned with a main scale mark, and that is the vernier scale reading. To find the actual reading, the user must then multiply the vernier scale reading with the vernier constant, which is provided by the manufacturer of the caliper and which can usually be found imprinted on the caliper, and then add the result with the main scale reading. Mathematically, actual reading = main scale reading + (vernier scale reading * vernier constant).