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The only troy weight in widespread use is the British Imperial troy ounce and its American counterpart. Both are based on a grain of 0.06479891 gram (exact, by definition), with 480 grains to a troy ounce (compared with 437 + 1 ⁄ 2 grains for an ounce avoirdupois).
The grain was the legal foundation of traditional English weight systems, [5] and is the only unit that is equal throughout the troy, avoirdupois, and apothecaries' systems of mass. [6]: C-6 The unit was based on the weight of a single grain of barley which was equal to about + 4 ⁄ 3 the weight of a single grain of wheat.
Another difference arose when Britain abolished the troy pound (373.241 7216 g) on 1 January 1879, leaving only the troy ounce (31.103 4768 g) and its decimal subdivisions, whereas the troy pound (of 12 troy ounces) and pennyweight are still legal in the United States, although they are no longer widely used.
Troy weight, avoirdupois weight, and apothecaries' weight are all built from the same basic unit, the grain, which is the same in all three systems. However, while each system has some overlap in the names of their units of measure (all have ounces and pounds), the relationship between the grain and these other units within each system varies.
Troy weights were used in England by jewellers. Apothecaries also used the troy pound and ounce, but added the drachms and scruples unit in the apothecaries' system of weights. Troy weight may take its name from the French market town of Troyes in France where English merchants traded at least as early as the early 9th century. [28]
troy weight, used for precious metals; avoirdupois weight, used for most other purposes; and; apothecaries' weight, now virtually unused since the metric system is used for all scientific purposes. The distinction between mass and weight is not always clearly drawn. Strictly a pound is a unit of mass, but it is commonly referred to as a weight.
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See Weight for detail of mass/weight distinction and conversion. Avoirdupois is a system of mass based on a pound of 16 ounces, while Troy weight is the system of mass where 12 troy ounces equals one troy pound. The symbol g 0 is used to denote standard gravity in order to avoid confusion with the (upright) g symbol for gram.