American customary system

The avoirdupois system (English: /ˌævərdəˈpɔɪz/; abbreviated avdp) is a measurement system of weights which uses pounds and ounces as units. It was first commonly used in the 13th century and was updated in 1959.

In 1959, by international agreement, the definitions of the pound and ounce became standardized in countries which use the pound as a unit of mass. The International Avoirdupois Pound was then created. It is the everyday system of weights used in the United States. It is still used, in varying degrees, in everyday life in the United Kingdom, Canada, and some other former British colonies, despite their official adoption of the metric system.

The avoirdupois weight system's general attributes were originally developed for the international wool trade in the Late Middle Ages, when trade was in recovery. It was historically based on a physical standardized pound or "prototype weight" that could be divided into 16 ounces. There were a number of competing measures of mass, and the fact that the avoirdupois pound had three even numbers as divisors (half and half and half again) may have been a cause of much of its popularity, so that the system won out over systems with 12 or 10 or 15 subdivisions. The use of this unofficial system gradually stabilized and evolved, with only slight changes in the reference standard or in the prototype's actual mass.

Over time, the desire not to use too many different systems of measurement allowed the establishment of "value relationships", with other commodities metered and sold by weight measurements such as bulk goods (grains, ores, flax) and smelted metals; so the avoirdupois system gradually became an accepted standard through much of Europe.

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