Advantage of vernier calipers
A vernier caliper consist 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.
Maybe the most distinct advantage of the vernier caliper is that it can provide very accurate measurements over a large range. It can be used for both intenal and external surfaces. In general, pocket models can measure from 0 to 3 inches, however, sizes are available all the way to 4 feet. You must be able to measure with a slide caliper and be able to read a vernier scale during use it.