Avoirdupois

Avoirdupois (abbreviated avdp.) is a weight measurement system built around pounds and ounces. It first came into common use in the 13th century CE and was updated in 1959. That year, an international agreement among countries using the pound as a mass unit fixed the International Avoirdupois Pound at exactly 0.45359237 kilograms. Today, it remains the everyday weight system in the United States, and it is still used to varying degrees in daily life in the United Kingdom, Canada, and some other former British colonies, even though those places have officially adopted the metric system.

The general features of the avoirdupois system were originally developed for the international wool trade during the Late Middle Ages, as trade was recovering. It was historically based on a physical standard pound—a prototype weight—that could be split into 16 ounces. Many competing mass measures existed at the time, and the avoirdupois pound’s ability to be divided evenly by half, then half again, and half again (giving three even divisors) likely boosted its popularity, helping it win out over systems with 12, 10, or 15 subdivisions. This unofficial system gradually stabilized and evolved, with only slight changes to the reference standard or the prototype’s actual mass. Over time, the desire to avoid juggling too many measurement systems led to the establishment of “value relationships” with other commodities—such as bulk goods (grains, ores, flax) and smelted metals—that were sold by weight. As a result, the avoirdupois system slowly became an accepted standard across much of Europe.

In England, King Henry VII authorized its use as a standard, and Queen Elizabeth I took action three times to enforce a common standard, laying the groundwork for what became the Imperial system of weights and measures. In the late 19th century, various governments independently redefined their base standards on a scientific basis and set ratios between local avoirdupois measurements and the international SI metric system. Because these legal actions were conceived separately, they did not always pick the same ratios to metric units for each avoirdupois unit. Consequently, after these standardizations, measurements with the same name often had slightly different recognized values in different regions (though the pound generally remained very similar). Since 1959, there has been no difference between the U.S. customary pound and the British imperial pound. An alternative mass system, the troy system—also using pounds and ounces—is generally reserved for precious materials.

**Etymology** The word *avoirdupois* comes from Anglo-Norman French *aveir de peis* (later *avoir du pois*), literally meaning “goods of weight.” The Old French *aveir*, as a verb meaning “to have” and as a noun meaning “property, goods,” derives from Latin *habere* (“to have, to hold, to possess something”); *de* means “from” or “of” (compare Latin); and *peis* means “weight,” from Latin *pēnsum*. This term originally referred to a class of merchandise—*aveir de peis*, “goods of weight”—things sold in bulk and weighed on large steelyards or balances. Only later did the term come to identify a particular system of units used for weighing such merchandise. Inconsistent spelling over time has left many variants, such as *haberty-poie* and *haber de peyse*. (The Norman *peis* became the Parisian *pois*; in the 17th century, *de* was replaced with *du*.) The current spelling of the last word in modern standard French is *poids*, but the spelling *avoirdupois* remained unchanged in the anglosphere.

**History** The rise of the measurement system corresponds to the revival of trade during the High Middle Ages after the early Crusades, when Europe saw towns grow, long-distance commerce replace warlord chaos, and annual fairs, tournaments, and trade by land and sea flourish. Two major hypotheses exist about the system’s origins. The older one suggests it began in France; a newer hypothesis holds that it is based on the weight system of Florence. The avoirdupois system is thought to have come into use in England around 1300, originally for weighing wool. In the early 14th century, several other specialized weight systems were in use, including the Hanseatic League’s system with a 16-ounce pound of 7200 grains and an 8-ounce mark. However, the main weight system—used for coinage and everyday purposes—was based on the 12-ounce tower pound of 5400 grains. From the 14th century until the late 16th century, the system’s basis and the prototype for today’s international pound, the avoirdupois pound, was also known as the wool pound or the avoirdupois wool pound. The earliest known version of the avoirdupois weight system had these units: a pound of 6992 grains, a stone of 14 pounds, a woolsack of 26 stone, an ounce of 1/16 pound, and the ounce divided into 16 “parts.” The earliest known occurrence of the word “avoirdupois” (or a variant) in England appears in a document called *Tractatus de Ponderibus et Mensuris* (“Treatise on Weights and Measures”). This document is listed in early statute books under the heading 31 Edward I, dated 2 February 1303. More recent statute books list it among statutes of uncertain date. Scholars today believe it was probably written between 1266 and 1303. Initially a royal memorandum, it eventually took on the force of law and was recognized as a statute by King Henry VIII and Queen Elizabeth I. In the *Tractatus*, the word “avoirdupois” refers not to a weight system but to a class of goods—specifically heavy goods sold by weight, as opposed to goods sold by volume, count, or another method. Since it is written in Anglo-Norman French, this document is not the first occurrence of the word in the English language.