Troy ounces
Troy weight is a system of units of mass customarily used for precious metals and gemstones. One troy ounce (abbreviated "oz" or "ozt") equals 31.1034768 grams exactly (or about 1.0971 oz. avoirdupois). There are only 12 troy ounces per troy pound, rather than the 16 ounces per pound found in the more common avoirdupois system. However, the avoirdupois pound has 7000 grains whereas the troy pound has only 5760 grains (i.e. 12 × 480 grains). Both systems use the same grain defined by the international yard and pound agreement of 1959 as exactly 0.06479891 grams. Therefore, the troy ounce is 480 grains (31.10 g), compared with the avoirdupois ounce, which is 437.5 grains (28.35 g). So the troy ounce is about 10% heavier (ratio 192/175) than the avoirdupois ounce, which is used for everyday purposes in the United States. Although troy ounces are still used to weigh gold, silver, and gemstones, troy weight is no longer used in most other applications. One troy ounce of gold is denoted with the ISO 4217 currency code XAU, while one troy ounce of silver is denoted as XAG.
Troy weight probably takes its name from the French market town of Troyes in France where English merchants traded at least as early as the early 9th century. The name "troy" is first attested in 1390, describing the weight of a platter, in an account of the travels in Europe of the Earl of Derby.
Charles Moore Watson (1844–1916) proposes an alternative etymology: The Assize of Weights and Measures (also known as Tractatus de Ponderibus et Mensuris), one of the statutes of uncertain date from the reign of either Henry III or Edward I, thus before 1307, specifies "troni ponderacionem"—which the Public Record Commissioners translate as "troy weight". The word "troni" refers to markets. Watson finds the dialect word "troi", meaning a balance in Wright's Dialect Dictionary. Troy weight referred to the tower system; the earliest reference to the modern troy weights is in 1414.
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