The vernier callipers

The vernier scale is a device that is used for gaining an extra decimal point in the measurement. A diagram is shown in Figure 1. To find the size of the object being measured you read:

(i) the zero position of the vernier 14.9 and a bit
(ii)then the line on the vernier scale that matches a main scale line as closely as possible in this case line 3

The dimension of the object is then 14.93 units.
Vernier scales of the type found in most school laboratories are usually used to measure objects up to a few centimetres across and to an accuracy of +/- 0.1 mm