History
Prior to the adoption of the metric system, many systems of troy weights were in use in various parts of Europe, among them Holland troy, Paris troy, etc. Their values varied from one another by up to several percentage points.
The only system of troy weights in widespread use today is the British Imperial, more commonly known as Imperial, and its American counterpart. Both the British Imperial and American troy weights are currently based on a grain of 0.06479891 gram (exact, by definition), with 480 grains to a troy ounce (compared with 437½ grains for an ounce avoirdupois). However, the British Empire abolished the 12-ounce troy pound in the 19th century, while it has been retained (although rarely used) in the American system.
In the USA, troy weights are part of the United States customary units.
The British Imperial system of weights and measures (also known as Imperial units) was established in 1824, prior to which the troy weight system was a subset of pre-Imperial English units.
Troy weights were first used in England in the 1400s, and were made official for gold and silver in 1527.
Read more about this topic: Troy Weight
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