A device used to allow very fine adjustments or very small measurements to be made. A vernier coupling, for example, has two sets of splines, or teeth. One set has 35 teeth and the other set has 36 teeth. When the coupling is moved ahead one tooth on the 35-tooth side, it moves ahead 10.285 degrees (360 ÷ 35 = 10.285). When it is moved back one tooth on the 36-tooth side, it moves back 10 degrees (360 ÷ 36 = 10).

By moving the coupling ahead one tooth on the 35-tooth side and back one tooth on the 36-tooth side, the shafts joined by the coupling have been moved by an amount of only 0.285 degree.