1 link of a chain = 7.92 inches. 25 links = 1 rod (or pole or perch) = 16.5 feet. 100 links = 1 chain = 66 feet = 4 poles (or 4 rods). 10 chains = 1 furlong = 660 feet = 40 poles. 80 chains = 1 mile. 10 sq. chains = 1 acre.
Chain
When the term is used without qualification, and always in the United States, the unit is Gunter's chain 1 (17th century — present ), = ⁄10 furlong = 4 perches= 22 yards = 66 feet = 20 .1168 meters. A square chain = 4,356 square feet (approximately 404.686 square meters).
Gunter’s chain is subdivided into 100 links; one link = 7 .92 inches. The square link is a unit of area, = 62.75 square inches (404.686 square centimeters).
In the English-speaking world, a unit of length used to measure land, at least as early as the 13th - 20th century , = 5.5 yards, (since 1959 in the United States and since 1963 in the UK, = 5 .0292 meters exactly). With metrification, the unit now survives as a legal measure only in the United States.
Rod
R. D. Connor (1987, page 39) makes a convincing argument, based on measurements of properties described in Saxon documents, that the rod is identical with the Saxon gyrd. Like Grierson, he speculates that the most probable origin of the gyrd's length is the length of twenty natural feet. (In 20th century American male median feet, that would be 208 inches, not much more than the rod's 198, and the feet of 11th century Englishmen were certainly smaller.)
In many contexts the rod is synonymous with the pole and the perch.
Statute mile
The word “mile” comes from the Latin mille passum , literally “thousand paces,” a unit introduced to Britain by the Roman occupation ( 57 BCE –450 CE ). Each passus consisted of five pes , the Roman foot, so the mille passum was 5,000 pes . This distance was also known as a milliarium , literally “milestone.” The mille passum was divided into 8 stadia , each of 625 pes . The Roman pes was shorter than our foot, and a well-accepted guess at the length of the mille passum in Roman Britain is about 1,479.5 meters, about 90% of a statute mile. The Saxons seem to have retained a 5,000-foot mile (their mil ), but the Saxon foot was even shorter than the Roman one, closer to the size of a real foot. The Saxon mil was probably about 1,257 meters, about 0.78 statute miles. The question then becomes how the mile grew from 5,000 feet to 5,280 feet. The answer seems to be that the English furlong became confused with the Roman stade . In those days legal proceedings, records and other official documents were kept in Latin. “Mile” in English naturally became “mille” in Latin. The nearest equivalent in Latin to the English “furlong” (660 feet), however, was the “ stade ” (625 pes ). The educated knew that the Roman mille passus contained 8 stadia , and continually translating “furlongs” as stadia planted the idea that a mile contained 8 furlongs, whereas in the past the two units had been used for entirely different purposes and had had no direct relationship. The result was confusion: 5,000-foot miles, 8- furlong miles, and even attempts to redefine the furlong to make 8 of them fit in a 5,000-foot mile. Something had to give. The length of the furlong, the basis of the acre, was not adjustable because the ruling classes' rents and revenues were based upon it, but a modest change in the mile would have no great impact. So Elizabeth I ended the confusion by coming down on the side of the 8-furlong, 5,280-foot mile, in effect abolishing the 5,000-foot mile.
Mille passus An ancient Roman unit of distance, literally, “a thousand passus,” where the passus is a unit conceptually equal to a pace of two steps. About 1478.5 meters.
Foot
A measure of length in the English-speaking world; since the 12th century , the foot has been legally defined as one-third of a yard. Since 1959 , one foot = 0 .3048 meter exactly; see yard and below.
The foot is only roughly the size of a human foot. The median foot of American males (as many smaller as bigger) is 10.4 inches long, and the female median is an inch smaller. Even 17-year old U.S. Army trainees have a median foot length of only 10.6 ″, and 95% of them have feet smaller than 11.2 ″.
The most recent change in the length of the foot was the result of an agreement among the English- speaking countries to eliminate discrepancies between their customary measures. The United States implemented the agreement by an announcement in the Federal Register on July 1, 1959 (“Refinement of Values for the Yard and the Pound”), and since then in the United States the foot has been the international foot = 0.3048 meter exactly. Since the United States is now almost the only country still using the foot, the name has become something of a joke.
U.S. Survey foot
The United States uses a different foot for one activity. When the United States adopted the international yard in 1959 , the U.S. Coast and Geodetic Survey, mappers of the nation, objected that converting all their geodetic data to international feet would be a horrendous undertaking. They were authorized to continue to use the previous definition of the foot, that of the Mendenhall order (U.S. Coast 1200 and Geodetic Survey Bulletin 26, April 5, 1893 ), one foot = /3937 meter. This foot is now known as the U.S. Survey foot, = 1.000 002 international feet, and is used only for land measurements. 1 m = 3.2808334366796 Survey ft
Acre In the English-speaking world, before 8th – 21st century , a unit of land area, now = 0.001 562 5 square miles = 160 square rods = 4,840 square yards = 43,560 square feet, about 0.404 685 64 hectare.
The acre was also used as a unit of tax assessment rather than of land area per se.
In the United States, since the acre is a land measure it is currently based on the U.S. survey foot and not on the international foot. One acre is about 4,046.873 square meters.
The acre was originally the amount of land that could be plowed in a single day with oxen, or actually, what could be done by midday, since refueling took all afternoon (the oxen had to be put out to pasture). Similar units of land area are found wherever animals are used for plowing; the German Morgen and Roman jugerum had much the same meaning. Like many units of land area, the acre was first thought of as a piece of land having certain dimensions. An acre was 40 perches long and 4 perches wide. (The king's perch was 5½ yards). A strip 40 perches long and 1 perch wide was a rood (not to be confused with the rod, a name (from the Saxon gyrd ) used by the 13th century for the perch.) Not until much later (the 16th century , according to R. M. Connor) did the acre begin to be thought of in geometric terms, as so many square feet or square rods.
The length of the acre, 40 perches, was roughly the distance a team of oxen could plow before needing a breather (this furrow-long became the furlong, 220 yards). Ploughmen prefer long furrows, as turning the team is a cumbersome process.
Regarding the width of the acre, 4 perches.
In actual use the size of the acre varied greatly, generally being larger in poor land than good. In some contexts it was almost synonymous with “small holding.”
Another complicating factor is that there were a variety of perches:
• a 13th century writer mentions perches of 18, 20, 22, and even 24 feet.
• an official report of 1820 found, besides the 16½ foot perch or rod, others of 18 feet, 21 feet, 24 feet, and 25 feet in use. The king's rod or perch, however, remained constant for centuries at 16½ feet.