Troyes

Troyes is a city in the Champagne region of France. In the twelfth and thirteenth centuries, the Champagne fairs — held at Troyes, Provins, Lagny, and Bar-sur-Aube — were the largest commercial gatherings in Europe. Merchants from Italy, Flanders, England, and Germany converged to trade wool, silk, spices, and precious metals. The problem of inconsistent weights demanded a solution. The troy weight system — weights agreed upon at Troyes — became the standard for precious metals.

A troy ounce is 31.1035 grams. An avoirdupois ounce is 28.3495 grams. The difference is about ten percent. A troy pound is 12 troy ounces (373.24 grams), while an avoirdupois pound is 16 avoirdupois ounces (453.59 grams). The systems are not interchangeable, and the troy system has no tons or hundredweight — it stops at the pound. It is a specialist system for weighing things that are too expensive to weigh carelessly.

England adopted troy weight for coinage and bullion under Henry VIII in the sixteenth century. The Royal Mint weighed silver and gold by the troy ounce. When the US Mint was established in 1792, it followed the British standard. Gold bars at Fort Knox are weighed in troy ounces. The London Bullion Market Association quotes gold and silver in troy ounces. Central banks around the world hold reserves measured in troy ounces.

The troy ounce outlasted the fairs that created it. Troyes is now a quiet cathedral city of 60,000 people. The great fairs ended in the fourteenth century when trade routes shifted. But every gold price you have ever seen — $2,000 per ounce, $1,800 per ounce — is a troy ounce. The medieval merchants at the Champagne fairs set the standard that still prices the world's gold.