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The pound or pound-mass (abbreviation: lb, lbm) is a measurement unit of mass used in the United States customary, Imperial, and other systems of measurement. The word pound is derived from the Latin phrase libra pondo for 'a pound weight' which is also the origin of the abbreviation lb. Many different definitions have been used, the most common today being the international avoirdupois pound of exactly 0.45359237 kilograms.

The common usage of the word pound to mean weight reflects the confusion between mass and weight resulting from the near uniformity of gravity on Earth. This accounts for the distinguishing terms pound-mass and pound-force. The pound-force is the force of 0.45359237 kilogram subject to a standard gravity of 9.80665 m/s², or approximately 4.448 Newtons.

History

The avoirdupois pound was defined by London merchants in 1303. By 1758, two standard weights for the avoirdupois pound existed, and when measured in troy grains they were found to be of 7,002 grains and 6,999 grains.

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 than it does now, of approximately 0.453592338 kg. This was a measured quantity, with the independently maintained artifact still serving as the official standard for that value, and was referred to as the Imperial pound. That value and the term Imperial pound are now essentially obsolete. In 1883, it was determined that 0.4535924277|u=kg was more accurate. With the Weights and Measures Act 1889, the United Kingdom legally defined the avoirdupois pound as the rounded value of 0.45359243 kg.

In the United States, the (avoirdupois) pound as a unit of mass has been officially defined in terms of the kilogram since the Mendenhall Order of 1893. In 1893, the relationship was specified to be 2.20462 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 United States and countries of the Commonwealth of Nations agreed upon common definitions for the pound and the yard. Since 1 July 1959, the international avoirdupois pound has been defined as exactly 0.45359237 kilogram.[8]

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 373.2417216 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.[citation needed] In the United Kingdom, the use of the troy pound was abolished on 6 January 1879.