Chain (unit)

chain
Unit system	imperial/US units
Unit of	length
Symbol	ch
1 ch in ...	... is equal to ...
imperial/US units	22 yd
metric (SI) units	20.1168 m

A chain (ch) is a unit of length. It measures 66 feet, or 22 yards, or 100 links, or 4 rods (20.1168 m). There are 10 chains in a furlong, and 80 chains in one statute mile. An acre is the area of 10 square chains (that is, an area of one chain by one furlong). The chain has been used for several centuries in Britain and in some other countries influenced by British practice.

By extension, chainage (running distance) is the distance along a curved or straight survey line from a fixed commencing point, as given by an odometer.

The chain was commonly used with the mile to indicate land distances and in particular in surveying land for legal and commercial purposes. Starting in the 19th century, the chain was used as a subdivision with the mile to show distances between railway stations, tunnels and bridges. In medieval times, local measures were commonly used, and many units were adopted that gave manageable units; for example the distance from London to York could be quoted in inches, but the resulting huge number would be unmemorable. The locally used units were often inconsistent from place to place.

In countries influenced by English practice, land plans prepared before about 1960 associated with the sale of land usually have lengths marked in chains and links, and the areas of land parcels are indicated in acres. A rectangle of land one furlong in length and one chain in width has an area of one acre.

In 1620, the clergyman Edmund Gunter developed a method of surveying land accurately with low technology equipment, using what became known as Gunter's chain; this was 66 feet long and from the practice of using his chain, the word transferred to the actual measured unit. His chain had 100 links, and the link is used as a subdivision of the chain as a unit of length.

American surveyors sometimes used a longer chain of 100 feet (30.48 m), also of 100 links, known as the engineer's chain or Ramsden's chain. The first such was constructed by Jesse Ramsden for the measurement of the Hounslow baseline at the start of the Anglo-French Survey (1784–1790). The term chain in this case usually refers to the measuring instrument rather than a unit of length; the distances measured with such an instrument are normally measured in feet (and usually decimal fractions of a foot, not inches).

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