The Chain is a tool used in Land Surveying to measure distance. The chain is a unit of length that is 66 ft or 22 yds. It is also said to be 100 links.
Ten chains lengths are a furlong, and 80 chains is one mile. An acre is the area of 10 square chains or 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.
In Texas and elsewhere in the Southwestern United States, the Vara Chain was used in surveying land Spanish land grants. The Vara Chain was made of 20 Vara; there were five links per Vara and each Vara is 33 1/3 inches long. This made the overall length 3333.3333333333 inches or 277.77 ft.
In North America the chain is still used in agriculture; measuring wheels use this unit of measurement and are still common and readily available in the United States and Canada. The chain is also used as the measure of the rate of spread of wildfires.
Under the U.S. Public Land Survey System, parcels of land are often described in terms of the section, quarter-section, and quarter-quarter-section. Respectively, these square divisions of land are approximately 80, 40 and 20 chains.
In the 19th century, the use of the chain was once very common in laying out townships and mapping the U.S. along the train routes. In the U.S. a federal law governing the Public Land Survey System, it is mandated that all official government surveys must be done with a surveyor's chain. In fact, distances on township plats made by the U.S. General Land Office are shown in chains.
Railroads in the United States have long since used decimal fractions of a mile, but the New York City Subway continues to use a chaining system using the 100 foot engineer's chain.
Also in the U.S., a modern variant of the chain is used in forestry for traverse surveys. This modern chain is a static cord, rope or line that is 50 meters in length, marked with a small tag at each meter, and also marked in the first meter every decimeter. When working in dense bush, a short axe or hatchet is commonly tied to the end of the chain, and thrown through the bush in the direction of the traverse, to ease working in dense forest.