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An old unit of area, equal to 160 square rods, 4,840 square yards, 43,560 sq. feet, or 4046.856 sq. meters. Ager in Latin, agros in Greek, ajras in Sanskrit, and acker in German all have the same meaning, namely, "a cultivated field". Most nations have, or had, some measure corresponding roughly with the area of land that could be plowed by a person in a day.

In England, the chain with which land was measured is 22 yards long (also the length of a cricket pitch). A square chain contains 22 × 2, or 484 sq. yards; so that 10 sq. chains make an acre. The acre is divided into 4 roods, a rood into 40 perches and a perch contains 20¼ sq. yards.

Before the fixing of the statute acre during the reign of George IV the acre varied in different parts of the country, from 2.115 statute acres in Cheshire to 0.477 in Leicestershire. The old Scottish acre was larger than the English, the Irish larger than the Scottish; 23 Scottish acres = 29 imperial acres; 30¼ Irish acres = 49 imperial.

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