furlong, old English unit of length, based on the length of an average plowed furrow (hence “furrow-long,” or furlong) in the English open- or common-field system. Each furrow ran the length of a 40 × 4-rod acre, or 660 modern feet. The standardization of such linear units as the yard, foot, and inch—begun by government enactment sometime between 1266 and 1303—recognized the traditional sizes of rods, furlongs, and acres as fixed and therefore simply redefined them in terms of the newly standardized units. Thus, the furlong, often measured as 625 northern (German) feet, became 660 standard English feet, and the mile, always 8 furlongs, became 5,280 feet. Today, the furlong is used almost exclusively in horse racing.