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.