Coins of the pound sterling another silver coin, the sceat, of 20 troy grains weight, which was in general circulation in Europe during the Middle Ages. The weight of the English penny was fixed at troy grains (about 1.46 grams) by Offa of Mercia, an 8th-century contemporary of Charlemagne. The coin's designated value, however, was that of 24 troy grains of silver (one pennyweight, or of a troy pound, or about 1.56 grams), with the difference being a premium attached by virtue of the minting into coins. Thus, 240 pennyweights made one troy pound of silver in weight, and the monetary value of 240 pennies
Troy weight by the international yard and pound agreement of 1959 as . Therefore, the troy ounce is , compared with the avoirdupois ounce, which is 437.5 grains or approximately 28.35 grams. The troy ounce, then, is about 10% heavier (ratio 192/175) than the avoirdupois ounce. Although troy ounces are still used to weigh gold, silver, and gemstones, troy weight is no longer used in most other applications. One troy ounce of gold is denoted with the ISO 4217 currency code XAU, while one troy ounce of silver is denoted as XAG. Troy weight probably takes its name from the French market