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 1/8th of the statute mile. “Furlong” was as early as the 9th century used to translate the Latin stadium, 1/8th of the Roman mile.