Definitions

Avoirdupois

Avoirdupois

[av-er-duh-poiz]
The avoirdupois (French avwaʀdypwɑ) system is a system of weights (or, properly, mass) based on a pound of sixteen ounces. 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.

History of the term

The word avoirdupois is from French and Middle English (Anglo-French) avoir de pois, "goods of weight" or "goods sold by weight", from Old French aveir de peis, literally "goods of weight" (Old French aveir, "property, goods", also "to have", comes from the Latin habere, "to have, to hold, to possess property"; de = "from", cf. Latin; peis = "weight", from Latin pensum). This term originally referred to a class of merchandise: aveir de peis, "goods of weight", things that were sold in bulk and were weighed on large steelyards or balances. Only later did it become identified with a particular system of units used to weigh such merchandise. The imaginative orthography of the day and the passage of the term through a series of languages (Latin, Anglo-French and English) has left many variants of the term, such as haberty-poie and haber de peyse. (The Norman peis became the Parisian pois. In the 17th century de was replaced with du.)

Original forms

These are the units in their original French forms:

Table of mass units
Unit Relative
value
Notes
dram or drachm 1/256 1/16 once
once 1/16
livre 1
quintal 100
tonne 2,000 20 quintaux

Note: The plural of quintal is quintaux.

British adaptation

When people in Britain began to use this system they 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 2,240 lb in order for masses to be easily converted between them and stones. The following are the units in the British or imperial adaptation of the avoirdupois system:

Table of mass units
Unit Relative
value
Metric
value
Notes
dram or drachm 1/256 ~1.772 g 1/16 oz
ounce (oz) 1/16 ~28.35 g 16 dr
pound (lb) 1 ~453.6 g 16 oz
stone (st) 14 ~6.35 kg ½ qtr
quarter (qtr) 28 ~12.7 kg 2 st
hundredweight (cwt) 112 ~50.8 kg 4 qtr
ton (t) 2,240 ~1,016 kg 20 cwt

Note: The plural form of stone is conventionally written the same as the singular when used after a number.

American customary system

The thirteen British colonies in North America (not including those that formed Canada), however, adopted the French system as it was. In the United States, quarters, hundredweights, and tons remain defined as 25, 100, and 2,000 lb respectively. The quarter is now virtually unused, as is the hundredweight outside of agriculture and commodities. If disambiguation is required then they are referred to as the "short" units, as opposed to the British "long" units.

Table of mass units
Unit Relative
value
Metric
value
Notes
dram (dr) 1/256 ~1.772 g 1/16 oz
ounce (oz) 1/16 ~28.35 g 16 dr
pound (lb) 1 ~453.6 g 16 oz
quarter (qtr) 25 ~11.34 kg ¼ cwt
hundredweight (cwt) 100 ~45.36 kg 4 qtr
ton (t) 2,000 ~907.2 kg 20 cwt

Internationalization

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. (See the Mendenhall Order for references.)

Due to the ambiguous meanings of "weight" as referring to both mass and force, it is sometimes erroneously asserted that the pound is only a unit of force. However, as defined above the pound is a unit of mass, which agrees with common usage. Also see pound-force and pound-mass.

See also

References

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