A troy ounce is 480 grains, somewhat heavier than an avoirdupois ounce (437.5 grains). A grain is defined to be exactly 64.79891 milligrams, hence 1 troy ounce is exactly 31.1034768 gramss, about 10% more than the avoirdupois ounce, which is about 28.3 g. There are also two versions of the fluid ounce, units of volume, of 28.4 ml and 29.6 ml.
In troy weight, there are 12 ounces in a pound, rather than 16 in the more common avoirdupois system. So a troy pound is 5760 grains (approx. 373.24 g), rather than 7000 (approx. 453.59 g).
This provides the technical curiosity that an ounce of gold weighs more than an ounce of feathers, but a pound of gold weighs less than a pound of feathers.
