The troy ounce is the unit of weight that runs the wholesale market for gold, silver, platinum, and palladium. One troy ounce weighs about thirty-one and one-tenth grams — to be exact, 31.1034768 grams. The name comes from the old market town of Troyes in France, where the unit was set down in writing long before there was a modern bullion trade. The system didn't die when the rest of commerce moved to the lighter avoirdupois ounce. It survived in the bullion and jewelry trades because the whole world of gold and silver had already learned to read a price in troy weight, and no one wanted to change the books.

The ounce the grocer weighs a cut of meat in is a different unit. The avoirdupois ounce weighs about twenty-eight and a third grams. That's the unit on the kitchen scale and the unit on the feed sack. It isn't the unit on a coin or a bar. A one-ounce gold eagle is a one troy ounce gold eagle. It holds more metal than an ounce pulled from a grocery scale. The gap is small but it's real. A reader of the market who doesn't know the gap can become confused when a bar and a coin seem to read different even though they bear the same marked weight.

Every spot price on a screen is a troy-ounce price. Every serious quote is read in the same unit.

Every price the household reader sees on a screen — on COMEX, in the London letter, or in a dealer quote — is a price for one troy ounce. When the ticker shows gold at three thousand two hundred dollars, it's three thousand two hundred dollars the troy ounce. Premiums are read against troy weight. Bar weights are cut to troy weight. Coin specifications are set in troy weight. The whole working language of the bullion market is spoken in this one unit.

For a household buyer, the working lesson is small and plain. When the ounce is named on a coin or a bar, it's a troy ounce. When the price is named on a screen, it's a troy ounce. The kitchen scale in the pantry isn't the scale that reads a bullion coin. A dealer worth the name will never quote a price in one unit and a weight in another. Every coin, every bar, every gram on the letter should read in the same troy system from the first number to the last. That's the plain rule of the house.