Named after Troyes, France, the troy system of weights was known to exist in medieval times. One cubic inch of distilled water, at 62 °F (17 °C), and at a barometric pressure of 30 inches of mercury, was determined to weigh 252.458 troy grains (gr).
There are 12 troy ounces per troy pound, rather than 16 avoirdupois ounces (oz) in the avoirdupois pound (lb) as in the more common avoirdupois system. The avoirdupois pound is 147⁄12 (≈ 14.583) troy ounces, since troy ounces are larger than avoirdupois ounces.
In Scotland the Incorporation of Goldsmiths of the City of Edinburgh used a system in multiples of sixteen. (See Assay-Master's Accounts, 1681-1702, on loan from the Incorporation to the National Archives of Scotland.) Thus there were 16 drops to the troy ounce, 16 ounces to the troy pound, and 16 pounds to the troy stone. The Scots had several other ways of measuring precious metals and gems, but this was the common usage for silver and gold.
|Troy pound (12 troy ounces)||5,760||373.241 721 6|
|Troy ounce (20 pennyweights)||480||31.103 476 8|
|Pennyweight||24||1.555 173 84|
|Grain||1||0.064 798 91|
The troy system was used in the Apothecaries' system, but with different further subdivisions.
http://www.troy-ounce.com - essay on the history of the Troy Ounce with many links to further reading.