Avoirdupois

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The avoirdupois system is a system of weights defining terms such as pound and ounce. It is the everyday system of weight used in the United States. It is still widely used by many people in Canada and the United Kingdom despite the official adoption of the metric system, including the compulsory introduction of metric units in shops. It is considered more modern than the alternative troy or apothecary or the medieval English mercantile and Tower systems. The name derives from the Old French term aveir de peis meaning literally "goods of weight", referring to goods sold by weight (as opposed to by the piece, for example).

In the avoirdupois system, all units are multiples or fractions of the pound, which is now defined as 0.45359237 kg in most of the English-speaking world since 1959.

These are the units in their original French forms:

16 drams/drachms = 1 once/ounce
16 onces = 1 livre/pound
25 livres = 1 quarter
4 quarters = 1 quintal/hundredweight
20 quintaux = 1 tonne/ton

People in Ireland and Britain, when they began to use this system, included the stone, which was eventually defined as fourteen avoirdupois pounds. The quarter, hundredweight, and ton were altered, respectively, to 28 lb, 112 lb, and 2240 lb in order for masses to be easily converted between them and stone. The British colonies in North America, however, adopted the system as it was. In the U.S., quarters, hundredweights and tons remain defined as 25, 100, and 2000 lb (though the two former are virtually unused); if disambiguation is required they are referred to as the "short" units, as opposed to the British "long" units.

The following are the units in the British or imperial adaptation of the avoirdupois system:

16 drams/drachms = 1 ounce (oz.)
16 ounces = 1 pound (lb.)
14 pounds = 1 stone (st.)
2 stone = 1 quarter (qtr.)
4 quarter = 1 hundredweight (cwt.)
20 hundredweight = 1 ton (t.)

Avoirdupois

See also