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A chain is a reliable machine component, which transmits power by means of tensile forces, and is used primarily for power transmission and conveyance systems. The function and uses of chain are similar to a belt.

In set theory, a chain is a totally ordered subset of a partially ordered set. In algebraic topology, a chain is a formal combination of simplices.

A chain is a measurement of length equivalent to 22 yards (20.12 metres), which is one tenth of a furlong or one eightieth of a mile. It was used in surveying land in England and places under English influence, such as the United States, but is obsolete in many places. It is also called the surveyor's chain or Gunter's chain after 17th century mathematican and astronomer Edmund Gunter, who introduced it. The distance of 22 yards is the length of the pitch (from wicket to wicket) in cricket.

The engineer's chain or Ramden's chain of 100 feet (30.48m) was also used.

See also: U.S. customary units