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.
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.