A vernier caliper has two main scales - one is known as the main scale and the other is the secondary scale, also known as the vernier scale. The secondary scale is the sliding scale, which moves over the main scale whenever the adjustable arm is moved. Both metric and inch measurements are provided on both of these scales.
Also, each scale is divided into 10 equal divisions and therefore has a least count of 0.1 mm each. While taking measurement, the movable arm is adjusted so as to fit the jaws around or inside of the object. Readings of both, main and vernier scales are important for making the final measurement. However, it is to note that readings in both the scales are taken in a different manner. The main scale reading is taken by looking where the mark, which is just on the left of 0, coincides on the vernier scale, whereas the secondary reading is taken by looking at that mark on the vernier scale which coincides or comes extremely close to a mark on the main scale. (Click the image below for an enlarged view of a reading.)
Reading a vernier caliper is easy and can be learned quickly. However, it requires sound concentration for reaching accurate measurement. Vernier calipers also have a locking nut provided right above the main scale. This helps in locking the movable arm at one particular place for attaining higher accurate measurement and for reducing the error rate.
It is also to note that vernier calipers are delicate instruments and utmost care should be taken while handling them. It is also important that a vernier caliper is well calibrated before use. This means that when the movable arm is brought to its original or closed position, the zero on the main scale should exactly coincide with the zero on the vernier scale. This is extremely important as the caliper measures even the slightest deviation and thus might provide an inappropriate final reading.