A vernier scale is an additional scale which allows a distance or angle measurement to be read more accurately than directly reading a uniformly-divided straight or circular measurement scale. It is a sliding secondary scale that is used to indicate where the measurement lies when it is 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.
When a measurement is taken by mechanical means using one of the above mentioned instruments, the measure is read off a finely marked data scale (the "fixed" scale, in the diagram). The measure taken will usually be between two of the smallest graduations on this scale. The indicating scale ("vernier" in the diagram) is used to provide an even finer additional level of accuracy without resorting to estimation.
Read more about Vernier Scale: History, Construction, Use, Examples, How A Vernier Scale Works, Vernier Acuity, Zero Error
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