5th period

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History
The name "furlong" derives from the Old English words furh (furrow) and lang (long). Dating back at least to the ninth century, 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 (22 yards) wide. For this reason, the furlong was once also called an acre's length, though in modern usage an acre can of course be any shape.
The furlong was historically viewed as equivalent to the Roman stade (stadium), which in turn derived from the Greek system. For example the King James bible will use the term "furlong" in place of the Greek "stadion", whereas modern translations will translate into miles in the main text and relate the (often very symbolic) original numbers in footnotes.
In the Roman system, there were 625 feet to the stade, eight stade 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 consisted of 1000 passus (5 feet, or double-step).
After the fall of Rome, Medieval Europe continued with the Roman system, which proceeded to "diversify" leading to serious complications in trade, taxation, etc. Around the turn of the century of 1300, England by decree standardized a long list of measures. Among the important units of distance and length at the time were foot, yard, rod, furlong and mile. The rod was 5½ yards (16½ feet), and the mile was eight furlongs, so the definition of the furlong became 40 rods and that of the mile became 5280 feet. (In other words, the reason the prime number 11 slipped into the English and subsequently the British and US systems was to accommodate the rod.)
The official use of furlong was abolished in the United Kingdom under the Weights and Measures Act 1985, which also abolished from official use many other traditional units of measurement.