A vernier scale is a device that lets the user measure more precisely than could be done unaided when reading a uniformly-divided straight or circular measurement scale. It is scale that indicates where the measurement lies in between two of the marks on the main scale. Verniers are common on sextants used in navigation, scientific instruments used to conduct experiments, machinists' measuring tools (all sorts, but especially calipers and micrometers) used to work materials to fine tolerances, and on theodolites used in surveying.
The vernier scale originated in ancient China as early as the Xin dynasty (9 AD).It was reinvented in its modern form in 1631 by the French mathematician Pierre Vernier (1580–1637). Its use was described in detail in English in Navigatio Britannica (1750) by John Barrow, the mathematician and historian.In some languages, this device is called a nonius and it was also commonly called a nonius in English until the end of the 18th century.Nonius is the Latin name of the Portuguese astronomer and mathematician Pedro Nunes (1502–1578) who in 1542 invented a related but different system for taking fine measurements on the astrolabe (nonius) that was a precursor to the vernier.The French astronomer Jérôme Lalande (1732–1807) popularized the name of the instrument as a ""vernier"" in his book on astronomy Traité d'astronomie (2 vols) (1764).
The main use of the vernier caliper is to measure the internal and the external diameters of an object. To measure using a vernier scale, the user first reads the finely marked ""fixed"" scale. This measure is typically between two of the scale's smallest graduations. The user then reads the finer vernier scale, which measures between the smallest graduations on the fixed scale—providing much greater accuracy.