The English first used the furlong in the 9th century or before. The name furlong comes from the Old English words furh (furrow) and lang (long). Originally it was the length of the furrow in one acre of a ploughed field.
An acre is an area that is one furlong long and one chain wide. For this reason, the furlong was once also called an acre's length. Around the year 1300, England standardised the furlong as 40 rods or 660 feet.
Distances for thoroughbred horse races in the United Kingdom, Ireland the United States, and Canada are still given in miles and furlongs. It is also used in the rail industry in the UK. Otherwise the unit is not used much.