A vernier caliper consists of an L-shaped member with a scale engraved on the long shank . A sliding member is free to move on the bar and carries a jaw that matches the arm of the L. The vernier scale is engraved on a small plate that is attached to the sliding member.
Perhaps the most distinct advantage of the vernier caliper over other types of calipers is the ability to provide very accurate measurements over a large range. It can be used for both internal and external surfaces . Pocket models usually measure from 0 to 3 inches, but sizes are available all the way to 4 feet, In using the vernier caliper, you must be able to measure with a slide caliper and be able to read a vernier scale.