The following was assembled from various sources and may, in places, be somewhat repetitive. This is intentional so as not to lose the context in which the various authors explained the history and use of "the chain."
A chain is a unit of length; it measures 66 feet or 22 yards (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.
The clergyman Edmund Gunter developed a method of surveying land accurately with low technology equipment, using what became known as Gunter's 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.
Land Surveyor's Chain (manufacturer's description of its product) - The ordinary Gunter's or surveyor's chain is sixty-six feet or four poles long, and is composed of one hundred links, connected each to each by two rings, and furnished with a tally mark at the end of every ten links. A link in measurement includes a ring at each end, and is seven and ninety two one hundredths inches long. In all the chains which we make the rings are oval and are sawed and well closed, the ends of the wore forming the hook being also filed and bent close to the link, to avoid kinking. The oval rings are about one third stronger than round ones.
Basic surveying devices and methods, including rope, poles, grape vines, and pacing, have existed for many years, but most of the tools commonly associated with surveying were developed during the 1600s. Three types of instruments were unique to North America: the compass, the transit (used for measuring straight lines), and the chain. The Gunter's Chain was used in America for nearly three hundred years and left a permanent mark on the profession.
The job of a surveyor in the eighteenth century was to measure land to be transferred from the crown to private ownership. When a warrant was issued from the secretary of state's office in Williamsburg, the county surveyor would survey the designated tract, draw a plat (a map showing the features of the land), and write a description of the land. Most surveyors learned their trade through an apprenticeship. County surveyors were appointed. Both surveyors and chain men (those who held the measuring chain) had to take an oath that they would be faithful, accurate, and would record their results without favor. However, many surveyors would add a link to their chains so as to be generous to the person obtaining the land. Surveying with modern techniques often shows the property to be larger than described.
During the 1700s and 1800s, Gunter's Chain was the standard for measuring distances and played a primary role in mapping out America. The chain consisted of 100 links and its total length was 4 poles (66 feet). Each link was connected to the next by a round ring. Eighty chains equaled one mile. Because the chains were hand-made, their measurements were rarely exact. Although the Gunter's Chain was the primary tool of surveyors to measure distance in North America from the 1600s to the end of the 1800s, it was eventually replaced by a more sophisticated and accurate instrument, the surveyor's tape. Surveyors today use electronic equipment, including the global positioning system.
The surveyor's chain pictured here (see above) was used by Peter McGee, Surveyor of Albemarle County, Virginia. also called Gunter's chain measuring device and arbitrary measurement unit still widely used for surveying in English-speaking countries. Invented by the English mathematician Edmund Gunter in the early 17th century, Gunter's chain was intended to be exactly 22 yards (about 20 m) long and divided into 100 links. In the device, each link is a solid bar. Measurement of the public land systems of the United States and Canada initially relied upon Gunter's chain.
When “Gunter's chain” is used without qualification, and always in the United States, the unit is (17th century – present), = 1/10 furlong = 4 perches= 22 yards = 66 feet = 20.1168 meters. All these units are based upon the survey foot, not the international foot. A square chain = 4,356 square feet (approximately 404.686 square meters). One link = 7.92 inches, a hundredth of a rod. The square link is a unit of area, = 62.75 square inches (404.686 square centimeters).
The Gunter’s chain—so named after its inventor—is generally used by the Land Surveyor. It is 66 ft. in length, each link measuring 7.92", and is very convenient when it is required to calculate areas in acres and decimals of an acre, since 10 sq. chains = 1 acre : also when linear dimensions are required in miles and furlongs, since 10 chains = 1 furlong and 80 chains =. 1 mile. When the term “chain” or “link” is used in a general sense, without reference to any particular unit of measurement, the Gunter's chain is inferred.
The surveyor's chain was called Gunter's, after its inventor, the English mathematician and astronomer Edmund Gunter (1581-1626).3 The length of the chain is 33 feet, or two poles (also called rods or perches) of 16-1/2 feet each. It consists of 50 links separated from one another by three rings. The length of a link, from the center of one connecting ring to the next, is 7.92 inches. Tally tags having one, two, three or four notches divide the chain into five sections, for convenience in measuring distances of less than a full chain. Gunter's chain served as the basic surveying instrument for three hundred years, until it was replaced in the early 20th century by the steel tape and later in the same century by the Global Positioning System.
Note that the above photographs are shown here courtesy of the following, which retain all rights:
Surveyors Chain: http://www.surveyhistory.org/surveyor
Colonial Williamsburg: http://www.history.org/history/teaching/enewsletter/volume3/december04/primsource.cfm
Encyclopedia Britannica: http://original.britannica.com/eb/article-9070475/surveyors-chain