The Troy Weight System
The system reserved for gold, silver and anything worth weighing precisely.
Origin
Named for the fairs of Troyes in Champagne, adopted in England for coinage by the fifteenth century and made the sole legal standard for precious metals in 1853.
The catch
A troy pound is lighter than a common pound, but a troy ounce is heavier. Twelve troy ounces make a troy pound; sixteen ordinary ounces make an ordinary one. So a pound of gold weighs less than a pound of feathers, while an ounce of gold weighs more.
Every gold price quoted anywhere in the world is per troy ounce, which makes this the most commercially significant of the three English weight systems despite being the least used. The grain is shared with avoirdupois and the apothecaries’ system, so all three agree at the bottom and diverge immediately above it. That shared foundation is what allowed merchants to convert between them without dispute.