A troy ounce, the only currently used unit of the system, is 480 grains, somewhat heavier than an avoirdupois ounce (437.5 grains). A grain is exactly 64.798 91 mg, hence one troy ounce is exactly 31.103 476 8 g, about 10 per cent more than the avoirdupois ounce, which is exactly 28.349 523 125 g. The troy ounce is the only ounce used in the pricing of precious metals, such as gold and silver, and this is the only remaining use of the troy ounce. In troy weight, there are 12 ounces in a pound, rather than 16 in the more-common avoirdupois system.
A troy pound is 5760 grains (about 373.24 g), rather than 7000 (about 453.59 g).
The use of troy weight for precious metals provides the technical curiosity that an ounce of gold has more mass than an ounce of feathers, but a pound of gold has less mass than a pound of feathers.
So that the poundtroy contains 5760 grains, of which 7000 are requisite to make the poundavoirdupois; of course the weight of the poundtroy is to that of the poundavoirdupois as 5760 to 7000, or as 144 to 175.
On the other hand, they have been used to weigh their money and medicine with the pennyweights and grains troy weight, and are not in the habit of using the pounds and ounces of that series.
It would be for their convenience, then, to suppress the pound and ouncetroy, and the drachm and quarter avoirdupois; and to form into one series the avoirdupoispound and ounce, and the troy pennyweight and grain.
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