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A furlong is a measure of distance in imperial units and U.S. customary units equal to one-eighth of a mile, equivalent to 220 yards, 660 feet, 40 rods, or 10 chains. Using the internationally accepted conversion ratio that one inch equals exactly 25.4 millimetres, one furlong is 201.168 metres. However, the United States does not uniformly use this conversion ratio. Older ratios are in use for surveying purposes in some states, leading to variations in the length of the furlong of about two parts per million, or 0.4 millimetres (1⁄64 inch). This variation is too small to have many practical consequences. Five furlongs are about 1.0 kilometre (1.00584 km is the exact value, according to the international conversion).
The name furlong derives from the Old English words furh (furrow) and lang (long). Dating back at least to early Anglo-Saxon times, it originally referred to the length of the furrow in one acre of a ploughed open field (a medieval communal field which was divided into strips). The system of long furrows arose because turning a team of oxen pulling a heavy plough was difficult. This offset the drainage advantages of short furrows and meant furrows were made as long as possible. An acre is an area that is one furlong long and one chain (66 feet or 22 yards) wide. For this reason, the furlong was once also called an acre's length, though in modern usage an area of one acre can be of any shape.
Among the early Anglo-Saxons, the rod was the fundamental unit of land measurement. A furlong was forty rods, an acre four by 40 rods, or four rods by one furlong. and thus 160 square rods. At the time, the Saxons used the North German foot, which was 10 percent longer than the foot of today. When England changed to the shorter foot in the late 13th century, rods and furlongs remained unchanged, since property boundaries were already defined in rods and furlongs. The only thing that changed was the number of feet and yards in a rod or a furlong, and the number of square feet and square yards in an acre. The definition of the rod went from 15 old feet to 16 1/2 new feet, or from five old yards to 5 1/2 new yards. The furlong went from 600 old feet to 660 new feet, or from 200 old yards to 220 new yards. The acre went from 36,000 old square feet to 43,560 new square feet, or from 4,000 old square yards to 4,840 new square yards.
The furlong was historically viewed as being equivalent to the Roman stade (stadium), which in turn derived from the Greek system. For example, the King James Bible uses the term "furlong" in place of the Greek stadion, although more recent translations often use miles or kilometres in the main text and give the original numbers in footnotes.
In the Roman system, there were 625 feet to the stadium, eight stadia to the mile, and three miles to the league. A league was considered to be the distance a man could walk in one hour, and the mile (from mille, meaning 1000) consisted of 1,000 passus (paces, five feet, or double-step).
After the fall of the Roman Empire, medieval Europe continued with the Roman system, which the people proceeded to diversify, leading to serious complications in trade, taxation, etc. Around the year 1300, by royal decree England standardized a long list of measures. Among the important units of distance and length at the time were the foot, yard, rod(or pole), furlong, and the mile. The rod was defined as 5 1/2 yards or 16 1/2 feet, and the mile was eight furlongs, so the definition of the furlong became 40 rods and that of the mile became 5,280 feet (eight furlongs/mile times 40 rods/furlong times 16 1/2 feet/rod).
A description from 1675 states, "Dimensurator or Measuring Instrument whereof the mosts usual has been the Chain, and the common length for English Measures four Poles, as answering indifferently to the Englishs Mile and Acre, 10 such Chains in length making a Furlong, and 10 single square Chains an Acre, so that a square Mile contains 640 square Acres." —John Ogilby, Britannia, 1675
The official use of the furlong was abolished in the United Kingdom under the Weights and Measures Act 1985, an act that also abolished the official use of many other traditional units of measurement.
In Burma, furlongs are currently used in conjunction with miles to indicate distances on highway signs.
In the rest of the world, the furlong has very limited use, with the notable exception of horse racing in most English-speaking countries, including Canada and the United States. The distances for horse-racing in Australia were metricized in 1972; but, in the United Kingdom, Ireland, Canada, and the United States, races are still given in miles and furlongs.
The city of Chicago's street numbering system allots a measure of 800 address units to each mile, in keeping with the city's system of eight blocks per mile. This means that every block in a typical Chicago neighborhood (in either North/South or East/West direction but rarely both) is approximately one furlong in length. Salt Lake City's blocks are also each a square furlong in the downtown area. The blocks become less regular in shape further from the center, but the numbering system (800 units to each mile) remains the same everywhere in Salt Lake County. Bus stops in Ann Arbor, Michigan, are about a furlong apart. Blocks in central Logan, Utah, and in large sections of Phoenix, Arizona, are similarly a square furlong in extent (eight to a mile, which explains the series of freeway exits: 19th Ave, 27th, 35th, 43rd, 51st, 59th ...). City blocks in the Hoddle Grid of Melbourne are also one furlong in length.
Much of Ontario, Canada, was originally surveyed on a ten-furlong grid, with major roads being laid out along the grid lines. Now that distances are shown on road signs in kilometres, it is obvious that these major roads are almost exactly two kilometres apart. The exits on highways running through Toronto, for example, are generally at intervals of two kilometres.
The furlong is also a base unit of the humorous FFF system of units.
The exact conversion of the furlong to SI units varies slightly among English-speaking countries. In Canada and the United Kingdom, which define the furlong in terms of the international yard of exactly 0.9144 metre, a furlong is 201.168 m. Australia does not formally define the furlong, but it does define the chain and link in terms of the international yard.
In the United States, which defines the furlong, chain, rod, and link in terms of the U.S. survey foot of exactly 1200⁄3937 metre, a furlong is approximately 201.1684 m long. The United States does not formally define a "survey yard". The difference of approximately two parts per million between the U.S. value and the "international" value is insignificant for most practical measurements.