vernier—An auxiliary scale sliding against and used in reading a primary scale. The total length of a given number of divisions on a vernier is equal to the total length of one more or one less than the same number of divisions on the primary scale. The vernier makes it possible to read a principal scale (such as a divided circle) much closer than one division of that scale. If a division on the primary scale is longer than a single division on the vernier, it is a direct vernier; if a division on the vernier is the longer, it is a retrograde vernier, so called because its numbers run in the opposite direction from those on the primary scale. The direct vernier is the usual type and is used in reading the circles on an engineer’s transit and on a repeating theodolite. Two verniers extending and numbered in opposite directions from the same index form a double vernier, used in reading a circle having graduations numbered in both directions. A single vernier so constructed and numbered that it may be read in either direction is termed a folding vernier.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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