Avoirdupois (;[1] abbreviated avdp.)[2] is a measurement system of weights that uses pounds and ounces as units.[3][4] It was first commonly used in the 13th century AD and was updated in 1959.[4]
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.[3] 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, New Zealand, Australia, 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.[a]
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.[3] The use of this unofficial system gradually stabilized[b] 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.[3]
In England, Henry VII authorized its use as a standard, and Queen Elizabeth I acted three times to enforce a common standard, thus establishing what became the Imperial system of weights and measures.[3]
Late in the 19th century various governments acted to redefine their
base standards on a scientific basis and establish ratios between local
avoirdupois measurements and international SI metric system standards.[3]
The legal actions of these various governments were independently
conceived, and so did not always pick the same ratios to metric units
for each avoirdupois unit. The result of this was, after these
standardisations, measurements of the same name often had marginally
different recognised values in different regions (although the pound
generally remained very similar). In the modern day, this is evident in
the small difference between United States customary and British
Imperial pounds.[3]
An alternative system of mass, the troy system, is generally used for precious materials. The modern definition of the avoirdupois pound (1 lb) is exactly 0.45359237 kilograms.[3][4]
Finely
crafted pan balance or scales with boxed set of standardized gram
weights sequenced in units of mass. Such scales are used to make the
most accurate of fine measurements, such as in the needs of empirical
chemistry.
Robust weights like these hexagonal decimal-scaled antiques were used for trade into the late 20th century.
https://en.wikipedia.org/wiki/Avoirdupois
