Vernier callipers

The Vernier calliper is used to measure the length or thickness of an object with great accuracy. It was invented by Pierre Vernier in 1630. It can measure length less than 1 mm.
A Vernier calliper consists of a fixed main scale or ordinary scale, a sliding Vernier scale, movable jaws, and a tail.
Parts of a Vernier Calipers

Main Scale: The main scale consists of a steel metallic strip graduated in centimeters at one edge and in inches at the other edge . It consists of a fixed external jaw and a fixed internal jaw.
Vernier Scale: A vernier scale  slides on the strip. It can be fixed in any position by the retainer. On the Vernier scale, 0.9 cm is divided into ten equal parts. It consisits of a fixed external jaw and a fixed internal jaw.

External Measuring Jaws: The outer measuring jaws (one of the main scale and the other of the vernier scale) helps to take the outer dimension of an object

Internal Measuring Jaws: The inner measuring jaws (one of the main scale and the other of the vernier scale) helps to take the inner dimension of an object.
Retainer: The retainer helps to retain the object within the jaws of the Vernier calipers to fix firmly.
Depth Measuring Strip: The depth measuring strip (prong) helps to measure the depth of an object. 

Least Count: The smallest length that can be measured accurately with a device is called the least count of that device. The least count of a Vernier calliper is known as the Vernier constant.

The least count or the smallest reading which you can get with the instrument can be calculated as:

Least Count = One main scale division - one Vernier scale division ------------ (i)