Imperial and US customary measurement systems in 1824, to take effect in 1826) and United States customary units which evolved from English Units from 1776 onwards. The earliest records of English units of measure involve the weight (and therefore the value) of Saxon coins. The penny introduced by Offa was about 20 grains (1.296 g). Edward the Elder increased the weight of the English penny to 26 grains (1.685 g), thereby aligning it with the penny of Charlemagne. By the time of the Norman Conquest (1066), it had decreased to 24 grains (1.555 g). This value was subsequently called the "pennyweight" and formed the basis of
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