vernier caliper, instrument for making very accurate linear measurements introduced in 1631 by Pierre Vernier of France. It uses two graduated scales: a main scale similar to that on a ruler and an especially graduated auxiliary scale, the vernier, that slides parallel to the main scale and enables readings to be made to a fraction of a division on the main scale. Vernier calipers are widely used in scientific laboratories and in manufacturing for quality control measurements.
In the , the vernier scale has 25 divisions, whereas the main scale has 24 divisions in the same length. This means that the divisions on the vernier scale are shorter than those on the main scale by 1/25 of a division on the main scale. In the figure, line 8 on the vernier coincides with line x on the main scale. To align lines 7 and y, the vernier would have to be moved to the left by 1/25 of a main-scale division; to align lines 6 and 40, the movement would be 2/25, and so on. By similar reasoning, the 0 line on the vernier would have to be moved a distance equal to 8/25 of a main-scale division to align it with the 8.50 line (before the 40 line, therefore this is the 38.50 line) on the main scale. This means that in the position shown in the figure the 0 line is 8/25 of a main-scale division to the right of the 8.50 line. The reading of the vernier is therefore 30 + 8.50 + 0.08 = 38.58.
Vernier scales are also made with 10 divisions in the same length as 9 on the main scale; the technique of reading such a scale is similar to that described above. A vernier scale can be used to increase the accuracy of angular as well as linear measurements.