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I did not know (but now I do!) that the acre is an area of length 1 furlong and a width of 1 chain. The Old English word "acre" means field and it was generally considered the area that could be ploughed in a day using a yoke or oxen. There are exactly 640 acres in a square mile. It is unusual in that it is not a measure of a square but a rectangle, even though most people in agriculture, including myself, use the unit regularly and conceptualize a square area.