1911 Encyclopædia Britannica/Furlong
FURLONG (from the O. Eng. furlang, i.e. “furrow-long”), a measure of length, originally the length of a furrow in the “common field” system. As the field in this system was generally taken to be a square, 10 acres in extent, and as the acre varied in different districts and at different times, the “furlong” also varied. The side of a square containing 10 statute acres is 220 yds. or 40 poles, which was the usually accepted length of the furlong. This is also the length of 18th of the statute mile. “Furlong” was as early as the 9th century used to translate the Latin stadium, 18th of the Roman mile.