A chain is a unit of length; it measures 66 feet or 22 yards or 100 links (20.1168m). There are 10 chains in a furlong, and 80 chains in one statute mile. An acre is the area of 10 square chains (that is, an area of one chain by one furlong). The chain has been used for several centuries in England and in some other countries influenced by British practice.
The chain was commonly used with the mile to indicate land distances and in particular in surveying land for legal and commercial purposes. In medieval times, local measures were commonly used, and many units were adopted that gave manageable units; for example the distance from London to York could be quoted in inches, but the resulting huge number would be unmemorable. The locally used units were often inconsistent from place to place.
In 1620, the clergyman Edmund Gunter developed a method of surveying land accurately with low technology equipment, using what became known as Gunters Chain; this was 66 feet long and from the practice of using his chain, the word transferred to the actual measured unit. His chain had 100 links, and the link is used as a subdivision of the chain as a unit of length.
In countries influenced by English practice, land plans prepared before about 1960 associated with the sale of land usually have lengths marked in chains and links, and the areas of land parcels is indicated in acres. A rectangle of land one furlong in length and one chain in width has an area of one acre. It is sometimes suggested that this was a medieval parcel of land capable of being worked by one man and supporting one family, but there is no documentary support for this assertion, and it would in any case have predated Gunter's work.
Extract of the original Wikipedia:Chain (length) article, used here to define terms used in some articles.