What is Avoirdupois?

The avoirdupois (IPA: /ˌævərdəˈpɔɪz/; French IPA: [avwɑrdypwɑ]) 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", and from Old French aveir de peis, literally "goods of weight", from aveir, "property, goods" (from aveir, "to have", from Latin habere, "to have, to hold, to possess property") + de, "from" (from the Latin) + peis, "weight", from Latin pensum, "weight". 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 1600s "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	1/16 livre
livre	1
quarter	25
quintal	100	plural: quintaux
tonne	2000

British adaptation

When people in Ireland and 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 2240 lb in order for masses to be easily converted between them and stone. 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	1/16 lb
pound (lb)	1	~453.6 g
stone (st)	14	~6.35 kg	14 lb. The plural form is conventionally written the same as the singular, 'stone'.
quarter (qtr)	28	~12.7 kg	2 st. Sometimes called the 'long quarter' to distinguish it from the U.S. unit.
hundredweight (cwt)	112	~50.8 kg	4 qtr. Sometimes called the 'long hundredweight' to distinguish it from the short ton.
ton (t)	2240	~1016 kg	20 cwt. Sometimes called the 'long ton' to distinguish it from the short ton.

American customary system

The British colonies in North America, however, adopted the French system as it was. In the U.S., quarters, hundredweights, and tons remain defined as 25, 100, and 2000 lb (though the quarter is virtually unused, as is the hundredweight outside of agriculture and commodities); if disambiguation is required then tons 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	1/16 lb
pound (lb)	1	~453.6 g
quarter (qtr)	25	~11.34 kg	25 lb. Not used to any significant extent.
hundredweight (cwt)	100	~45.36 kg	4 qtr. Sometimes called the 'short hundredweight' to distinguish it from the long hundredweight.
ton (t)	2000	~907.2 kg	20 cwt. Sometimes called the 'short ton' to distinguish it from the long ton.

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)

Further, these weights were considered units of force, not mass. Hence in planetariums one amusement for the audience was a series of scales to show how one's weight would be different on planets with different surface gravity. The use of the metric units as primary has brought about the pronouncement that the avoirdupois units are now masses as well.

See also

References