vernier caliper

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A measuring instrument consisting of an L-shaped frame with a linear scale along its longer arm and an L-shaped sliding attachment with a vernier, used to read directly the dimension of an object represented by the separation between the inner or outer edges of the two shorter arms.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a type of caliper using the vernier scale, an extremely precise measuring instrument

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a caliper with a vernier scale for very fine measurements