chain

(ch)

   a unit of distance used or formerly used by surveyors. Although the unit is not often used today, measured distance along a road or railroad is commonly called chainage regardless of the units used. The traditional British surveyor's chain, also called Gunter's chain because it was introduced by the English mathematician Edmund Gunter (1581-1626) in 1620, is 4 rods¹ long: that's equal to exactly 1/80 mile, 1/10 furlong, 22 yards, or 66 feet (20.1168 meters). The traditional length of a cricket pitch is 1 chain. Gunter's chain has the useful property that an acre is exactly 10 square chains. The chain was divided into 100 links. American surveyors sometimes used a longer chain of 100 feet, known as the engineer's chain or Ramsden's chain. (However, Gunter's chain is also used in the U.S.; in fact, it is an important unit in the Public Lands Survey System.) In Texas, the vara chain of 2 varas (55.556 ft) was used in surveying Spanish land grants. In the metric world, surveyors often use a chain of 20 meters (65.617 ft). See also shackle and shot² for anchor chain lengths.