Troy weight is a system of units of mass

Mass

Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 customarily used for precious metal

Precious metal

A precious metal is a rare, naturally occurring metallic chemical element of high economic value.Chemically, the precious metals are less reactive than most elements, have high lustre, are softer or more ductile, and have higher melting points than other metals...

s, gemstone

Gemstone

A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...

s, and black powder.
There are 12 troy ounces per troy pound, rather than the 16 ounces per pound found in the more common avoirdupois system

Avoirdupois

The avoirdupois system is a system of weights based on a pound of 16 ounces. It is the everyday system of weight used in the United States and is still widely used to varying degrees by many people in Canada, the United Kingdom, and some other former British colonies despite the official adoption...

. The troy ounce is 480 grains, compared with the avoirdupois ounce, which is 437-1/2 grains. Both systems use the same grain of exactly 0.06479891 gram. Although troy ounces are still used to weigh gold, silver and gemstones, the troy pound is no longer used.

Etymology

The name "troy" is first attested in 1390. Troy weight probably takes its name from the French market town of Troyes

Troyes

Troyes is a commune and the capital of the Aube department in north-central France. It is located on the Seine river about southeast of Paris. Many half-timbered houses survive in the old town...

 in France where English merchants traded at least as early as the early 9th century.

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 437 1/2 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

United States customary units

United States customary units are a system of measurements commonly used in the United States. Many U.S. units are virtually identical to their imperial counterparts, but the U.S. customary system developed from English units used in the British Empire before the system of imperial units was...

.

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.

The origin of the troy weight system is unknown. Though the name probably comes from the Champagne fairs

Champagne fairs

The Champagne fairs were an annual cycle of trading fairs held in towns in the Champagne and Brie regions of France in the Middle Ages. From their origins in local agricultural and stock fairs, the Champagne fairs became an important engine in the reviving economic history of medieval Europe,...

 at Troyes

Troyes

Troyes is a commune and the capital of the Aube department in north-central France. It is located on the Seine river about southeast of Paris. Many half-timbered houses survive in the old town...

, in northeastern France, the units themselves are probably of more northern origin. English troy weights were apparently derived from the nearly identical troy weight system of Bremen. (The Bremen troy ounce had a mass of 480.8 British Imperial grains.

Units of measurement

Troy pound (troy)
The troy pound (troy) is 5,760 grains (≈ 373.24 g, 12 oz t), while an avoirdupois

Avoirdupois

The avoirdupois system is a system of weights based on a pound of 16 ounces. It is the everyday system of weight used in the United States and is still widely used to varying degrees by many people in Canada, the United Kingdom, and some other former British colonies despite the official adoption...

 pound is ≈21.53% heavier at 7,000 grains (≈ 453.59 g).

Troy ounce (oz t)
One troy ounce (oz t) is equal to 31.1034768 grams.

Pennyweight (dwt)
The pennyweight symbol is dwt. There are 24 grains in 1 dwt, and 20 dwt in one troy ounce. Because there were 12 troy ounces in the old troy pound, there would have been 240 pennyweights to the pound—the basis of the fact that the old British pound (currency)

Pound sterling

The pound sterling , commonly called the pound, is the official currency of the United Kingdom, its Crown Dependencies and the British Overseas Territories of South Georgia and the South Sandwich Islands, British Antarctic Territory and Tristan da Cunha. It is subdivided into 100 pence...

 contained 240 pence. (However, prior to 1526, English pound sterling

Pound sterling

The pound sterling , commonly called the pound, is the official currency of the United Kingdom, its Crown Dependencies and the British Overseas Territories of South Georgia and the South Sandwich Islands, British Antarctic Territory and Tristan da Cunha. It is subdivided into 100 pence...

 was based on the tower pound, which is of a troy pound.) The d in dwt stands for denarius

Denarius

In the Roman currency system, the denarius was a small silver coin first minted in 211 BC. It was the most common coin produced for circulation but was slowly debased until its replacement by the antoninianus...

, the ancient Roman coin, referred to in the New Testament, that equates loosely to a penny. The symbol d for penny can be recognized in the old-style notation for British pennies, in which a quantity of money expressed in pounds, shillings, and pence was indicated using the symbols £, s, and d, respectively. For example, £6.11s.8d indicated six pounds, eleven shillings, and eight pence.

Troy grain

Scottish system

In Scotland, the Incorporation of Goldsmiths of the City of Edinburgh used a system in multiples of sixteen. (See Assay-Master's Accounts, 1681–1702, on loan from the Incorporation to the National Archives of Scotland.) Thus, there were 16 drops to the troy ounce, 16 ounces to the troy pound, and 16 pounds to the troy stone. The Scots had several other ways of measuring precious metals and gems, but this was the common usage for gold and silver.

Conversions

Unit	Grains	Grams
Troy pound (12 troy ounces)	5,760	373.241 721 6
Troy ounce (20 pennyweights)	480	31.103 476 8
Pennyweight	24	1.555 173 84
Grain	1	0.064 798 91

The troy system was used in the apothecaries' system

Apothecaries' system

The apothecaries' system of weights is a historical system of mass units that were used by physicians and apothecaries for medical recipes, and also sometimes by scientists. The English version of the system is closely related with the English troy system of weights, the pound and grain being...

, but with different further subdivisions.

Relationship to British coinage

The troy system was the basis for the pre-decimalisation British system of coinage introduced by Henry II of England

Henry II of England

Henry II ruled as King of England , Count of Anjou, Count of Maine, Duke of Normandy, Duke of Aquitaine, Duke of Gascony, Count of Nantes, Lord of Ireland and, at various times, controlled parts of Wales, Scotland and western France. Henry, the great-grandson of William the Conqueror, was the...

, in which the penny

Penny

A penny is a coin or a type of currency used in several English-speaking countries. It is often the smallest denomination within a currency system.-Etymology:...

 was literally one pennyweight

Pennyweight

A pennyweight is a unit of mass that is equal to 24 grains, 1/20 of a troy ounce, 1/240 of a troy pound, approximately 0.054857 avoirdupois ounce and exactly 1.55517384 grams....

 of silver. One pound sterling

Pound sterling

The pound sterling , commonly called the pound, is the official currency of the United Kingdom, its Crown Dependencies and the British Overseas Territories of South Georgia and the South Sandwich Islands, British Antarctic Territory and Tristan da Cunha. It is subdivided into 100 pence...

 was equal to twenty shilling

Shilling

The shilling is a unit of currency used in some current and former British Commonwealth countries. The word shilling comes from scilling, an accounting term that dates back to Anglo-Saxon times where it was deemed to be the value of a cow in Kent or a sheep elsewhere. The word is thought to derive...

s, with each shilling equal to twelve pennies

Penny

A penny is a coin or a type of currency used in several English-speaking countries. It is often the smallest denomination within a currency system.-Etymology:...

. Thus, one pound sterling equals 240 pennyweights, or one troy pound of sterling silver

Sterling silver

Sterling silver is an alloy of silver containing 92.5% by mass of silver and 7.5% by mass of other metals, usually copper. The sterling silver standard has a minimum millesimal fineness of 925....

.