A chain is a surveyors unit of length that equals 66 feet. There are 80 chains in a mile. An acre is 1 chain by 10 chains. The convenience of the chain for land measure arises from the fact that an acre is one furlong (a "furrow long") by one chain, and there are 10 chains in a furlong.
Early surveyors used a Gunter's chain to measure distance. The chain, invented in 1620 by Edmund Gunter, consisted of 100 links, each of which was 0.66 feet long. The chain came to be replaced by a metal tape, but the term "chaining" was still used to refer to the process of measuring distance by use of the tape.