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The troy weight units are the grain, the pennyweight (24 grains), the troy ounce (20 pennyweights), and the troy pound (12 troy ounces). The troy grain is equal to the grain unit of the avoirdupois system, but the troy ounce is heavier than the avoirdupois ounce, and the troy pound is lighter than the avoirdupois pound. Legally, one troy ounce ...
A troy pound (abbreviated lb t [26]) is equal to 12 troy ounces and to 5,760 grains, that is exactly 373.241 7216 grams. [27] 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.
The troy pound (5760 grains) was used for precious metals, the apothecaries' pound, (also 5760 grains) was used by pharmacists and the avoirdupois pound (7000 grains) was used for general purposes. The apothecaries and troy pounds are divided into 12 ounces (of 480 grains) while the avoirdupois pound has 16 ounces (of 437.5 grains).
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.
To unify all weight systems used by apothecaries, the Irish pharmacopœia of 1850 introduced a new variant of the apothecaries' system which subdivided a new apothecaries' pound of 12 avoirdupois ounces instead of the troy pound. [12] To allow effective use of the new system, new weight pieces were produced.
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A troy ounce (abbreviated oz t) is equal to 480 grains. Consequently, the international troy ounce is equal to exactly 31.1034768 grams. There are 12 troy ounces in the now obsolete troy pound. Today, the troy ounce is used only to express the mass of precious metals such as gold, platinum, palladium, rhodium or silver.