a traditional English weight system of great antiquity, apparently in use since long before the Norman conquest of 1066. The system is believed to be named for the French market town of Troyes, where English merchants traded at least as early as the time of Charlemagne (early ninth century).

The system is based on the troy pound of 5760 grains.
The pound was divided into 12 ounces (480 grains)
each containing 20 pennyweight (24 grains).

Apothocaries, however,
divided the troy ounce into 8 drams (60 grains) each containing 3 scruples (20 grains).

The origin of the troy system is not clear, but a number of scholars believe the dram corresponds to the denarius, a Roman coin that weighed about 60 English grains and (when used as a weight) was also divided into 3 scruples.

The troy system was always the theoretical basis of the traditional English monetary system, in which there were 12 pence (pennies) to the shilling and 20 shillings to the pound.

However, in medieval England pennies did not actually weigh a troy pennyweight, because they were made using the tower weight system (see above) and thus weighed 22.5 grains instead of 24.

In 1527, Henry VIII abolished the tower pound and made the troy system official for coinage; thereafter silver shillings weighed exactly 0.6 troy ounce. The smaller troy weights continued in common use in pharmacy and monetary affairs into the early twentieth century, but the troy pound was abolished in 1878 to avoid any commercial confusion with the avoirdupois pound. The troy system is practically obsolete today, but the prices of precious metals are still quoted by the troy ounce.

grain (gr)
a traditional unit of weight. The grain, equal to 1/480 troy ounce (see also pound [2]), or exactly 64.798 91 milligrams, was the legal foundation of traditional English weight systems, with various pounds being defined as a specified number of grains: 5760 grains in a troy pound and 7000 grains in an avoirdupois pound, for example. In the version of the troy system used by jewelers, there are 24 grains in a pennyweight and 20 pennyweight in an ounce. In the version used by apothecaries, there are 20 grains in a scruple, 3 scruples in a dram, and 8 drams in an ounce. Originally the grain was defined in England as the weight of a barleycorn. This made the English grain larger the corresponding grain units of France and other nations of the Continent, because those units were based on the weight of the smaller wheat grain.

Units of Weight