Gunter’s chain is a surveying instrument with 100 links. It is 22 yards (4 rods or 1/10 of a furlong) long, which is 1/80 mile. One square chain is 66 feet by 66 feet (4,356 square feet). One acre is 10 square chains, equal to 43,560 square feet or 1/640 of a square mile.