The pound's abbreviation lb comes from a Roman word meaning balance
Why is pound written lb? Because the unit descends from the Roman libra, a word for scales, and its full phrase libra pondo, weight measured by the balance, gave English the word pound. Since 1959 the international pound has been fixed at exactly 0.45359237 kilograms, a number chosen with surprising care.
Germanic languages borrowed the Latin phrase long ago, which is why German Pfund, Dutch pond and Swedish pund are cousins of pound. The Roman libra itself weighed about 328.9 grams and split into 12 unciae, the ancestor of the ounce. Calling the unit simply a pound also reflects an old blurring of mass and weight, which is why physicists now distinguish pound-mass from pound-force.
Britain juggled several pounds. King Offa of Mercia, who reigned from 757 to 796, introduced a money system based on a Saxon pound of silver, later called the tower pound. Henry VIII's parliament switched the coinage standard to the troy pound in 1528, and troy ounces still rule the trade in gold and silver, assumed whenever an ounce type goes unstated. The avoirdupois or wool pound came into general use around 1300 at 6,992 grains and was redefined under Elizabeth I as 7,000. By 1758, two official Elizabethan standards weighed 7,002 and 6,999 grains.
The modern pound came from an agreement between the United States and Commonwealth countries, effective 1 July 1959, and Britain wrote it into the Weights and Measures Act of 1963. Because a pound holds exactly 7,000 grains, the kilogram conversion was picked to divide cleanly by seven with a terminating decimal, making a grain exactly 64.79891 milligrams.
Metrication was supposed to finish it off. In the UK, European rules looked set to banish pounds and ounces, but in 2007 the European Commission dropped the demand for metric-only packaging and allowed dual labels indefinitely. America's Metric Conversion Act of 1975 named metric the preferred system without outlawing customary units, and the US remains the only industrialised country whose commerce does not mainly run on metric.
Source: Pound (mass)
