Pound (mass)
The pound or pound-mass (abbreviation: lb, or sometimes in the United States: #) is a unit of mass used in the imperial, United States customary and other systems of measurement. A number of different definitions have been used, the most common today being the international avoirdupois pound of exactly 453.59237 grams.
The word pound comes from the Latin word pendere, meaning "to weigh".  The Latin word libra means "scales, balances" and it also describes a Roman unit of mass similar to a pound.  This is the origin of the abbreviation "lb" for the pound.

# Definitions
Historically, in different parts of the world, at different points in time, and for different applications, the pound (or its translation) has referred to broadly similar but not identical standards of mass or weight.

## English pounds
A number of different definitions of the pound have been used in England.  Amongst these are the avoirdupois pound and the obsolete Troy, tower, merchant's and London pounds.
There is a historical link between the pound as a unit of mass and the pound as a unit of currency.  Originally the pound sterling was equivalent to the value a Tower pound of silver (worth about 79 or about $158 US today).  In 1528, the standard was changed to the Troy pound (worth about 84 or $168 today).

### Avoirdupois pound
The avoirdupois pound was invented by London merchants in 1303.  Originally it was based on independent standards measured to be about 7,002 troy grains.
During the reign of Henry VIII of England, the avoirdupois pound was redefined as 7,000 troy grains. Since then, the grain has often been considered as a part of the avoirdupois system.
In the United Kingdom, the avoirdupois pound was defined as a unit of mass by the Weights and Measures Act of 1878, but having a different value (in relation to the kilogram) than it does now, of approximately 0.453592338 kg, which would make the kilogram approximately equal to 2.20462278 pounds. (This was a measured quantity, with the independently maintained artifact still serving as the official standard for this pound.) This old value is sometimes called the imperial pound, and this definition and terminology are obsolete unless referring to the slightly-different 1878 definition.
In the United States, the (avoirdupois) pound as a unit of mass has been officially defined in terms of the kilogram since 1893.  In 1893, the relationship was specified to be 2.20462 pounds per kilogram.  In 1894, the relationship was specified to be 2.20462234 pounds per kilogram.  This change followed a determination of the British pound.  The current international pound differs from the United States 1894 pound by approximately one part in 10 million.
In 1958 the United States and countries of the Commonwealth of Nations agreed upon common definitions for the pound and the yard.  The international avoirdupois pound was defined as exactly 453.59237 grams.
In the United Kingdom, the use of the international pound was implemented in the Weights and Measures Act 1963.
An avoirdupois pound is equal to 16 avoirdupois ounces and to exactly 7,000 grains.  The conversion factor between the kilogram and the international pound was therefore chosen to be divisible by 7, and an (international) grain is thus equal to exactly 64.79891 milligrams.

### Troy pound
The troy pound takes its name from the French market town of Troyes in France where English merchants traded at least as early as the time of Charlemagne (early ninth century).  The system of Troy weights was used in England by apothecaries and jewellers.
A troy pound is equal to 12 troy ounces and to 5,760 grains. Today, the grain is common to the avoirdupois and troy systems of units of mass making an international troy pound equal to 373.241721 grams.
The troy pound is no longer in general use. In Canada, Australia, the United Kingdom, and other places the troy pound is no longer a legal unit for trade. In the United Kingdom, the use of the troy pound was abolished on 6 January 1879. The troy ounce is still used for measurements of precious metals such as gold, silver, and platinum, and sometimes gems such as opals.
Most measurements of the mass of precious metals using pounds refer to troy pounds, even though it is not always explicitly stated that this is the case.  Some notable exceptions are:
- Encyclopdia Britannica which uses either avoirdupois pounds or troy ounces, likely never both in the same article, and
- the mass of Tutankhamun's sarcophagus lid.  This is 110 kilograms.  It is often stated to have been 242 or 243 (avoirdupois) pounds but sometimes, much less commonly, it is stated as 296 (troy) pounds.

### Tower pound
The tower pound was based on the wheat grain unlike all the other English measures, where the grain was based on the barley grain.

### Merchants'  pound
The merchants' pound (mercantile pound, libra mercantoria or commercial pound) was equal to 9,600 wheat grains (15 tower ounces or 6,750 grains).  It was used in England until the 14th century for most goods (other than money, spices and electuaries).

### London pound
A London pound was equal to 7,200 troy grains (16 tower ounces or, equivalently, 15 troy ounces).

### Wool pound
The wool pound was equal to 6,992 grains.  It was a unit of mass used to measure the quantity of wool.

## Roman libra
The libra (Latin for "pound") is an ancient Roman unit of mass that was equivalent to approximately 327 grams.  It was divided into 12 uncia, or ounces.  The libra is the origin of the abbreviation for pound, lb.

## French livre
Since the Middle Ages various pounds have been used in France.  The word pound translates to  livre in French, a word which continues to be used today to refer to a metric pound.
The livre esterlin was equivalent to about 367.1  (Expression error: Missing operand for *. ) and was used between the late ninth and the mid-fourteenth centuries.
The livre poids de marc or livre de Paris was equivalent to about 489.5 grams (7,555 gr) and was used between the 1350s and the late 18th century..  It was introduced by the government of John II.
The livre metrique was set equal to the kilogram by the decree of 13 Brumaire an IX between 1800 and 1812.  This was a form of official metric pound.
The livre usuelle was set equal to 500 grams, by the decree of 28 March 1812.  It was abolished as a unit of mass effective 1 January 1840 by a decree of 4 July 1837.

## Russian funt
The Russian pound (, funt) is an obsolete Russian unit of measurement of mass. It is equal to 409.5124 g.

## Jersey pound
A Jersey pound is an obsolete unit of mass used on the island of Jersey from the 14th century to the 19th century. It was equivalent to about 7,561 grains. It may have been derived from the French livre poids de marc.

## Trone pound
The trone pound is one of a number of obsolete Scottish units of measurement. It was equivalent to between 21 to 28 avoirdupois ounces.

## Metric pounds
In many countries upon the introduction of a metric system, the pound (or its translation) became an informal term for half of a kilogram or 500 grams, often following an official redefinition of an existing unit during the 19th century.  The Dutch pond is an exception. It was officially redefined as 1 kilogram, with an ounce of 100 grams.  If the pound is used in the Netherlands today it is likely to refer to 500 grams; the former definition is no longer used.  However, the 100-gram ounce remains in limited use. In daily life pond is exclusively used for amounts of 500-grams, as is ons for 100 grams.
In German the term is Pfund, in French livre, in Dutch pond, in Spanish and Portuguese libra, and in Italian libbra.
Hundreds of older pounds were replaced in this way.  Examples of the older pounds are one of around 459 to 460 grams in Spain, Portugal, and Latin America; one of 498.1 grams in Norway; and several different ones in what is now Germany.
Although the use of the pound as an informal term persists in these countries to a varying degree, scales and measuring devices are denominated only in grams and kilograms.  A pound of product must be determined by weighing the product in grams.  The use of the term pound is usually forbidden for official use in trade.

# Use in commerce
In the United States of America the United States Department of Commerce, the Technology Administration, and the National Institute of Standards and Technology (NIST) have defined the use of mass and weight in the exchange of goods under the Uniform Laws and Regulations in the areas of legal metrology and engine fuel quality in NIST Handbook 130.
NIST Handbook 130 states:
U.S. federal law, which supersedes this handbook, also defines weight, particularly Net Weight, in terms of the avoirdupois pound or mass pound.  From 21CFR101 Part 101.105 - Declaration of net quantity of contents when exempt:
See also 21CFR201 Part 201.51 - "Declaration of net quantity of contents" for general labeling and prescription labeling requirements.
From paragraph "a" above, although the avoirdupois pound is a measure of mass, in commerce it is used with the term "Net Weight", because "there is a firmly established general consumer usage and trade custom of declaring the contents of a liquid by weight, or a solid..."