A device for obtaining accurate measurements of length, to 1/10th, 1/100th or smaller fractions of a unit. It consists of a fixed graduated main scale against which a shorter vernier scale slides. The vernier scale is graduated into divisions equal to nine-tenths of the smallest unit marked on the main scale. The vernier scale is often adjusted by means of a screw thread. A reading is taken by observing which of the markings on the scales coincide.