unit of avoirdupois weight, equal to 16 ounces, 7,000 grains, or 0.4536 kg, and of troy (troy weight) and apothecaries' weight, equal to 12 ounces, 5,760 grains, or 0.37 kg. The Roman ancestor of the modern pound, the libra, is the source of the abbreviation lb. In medieval England several derivations of the libra vied for general acceptance. Among the earliest of these, the Tower pound, so called because its standard was kept in the Royal Mint in the Tower of London (London, Tower of), was applied to precious metals and drugs and contained 5,400 grains, or 0.350 kg, while the mercantile pound contained 6,750 grains, or 0.437 kg. The troy pound, believed to have originated in Troyes, France, superseded the lighter Tower pound in 1527 as the gold and silver standard. Increased trade with France led also to the adoption of the 16-ounce avoirdupois pound in the 16th century to replace the mercantile pound.

The British monetary pound (pound sterling) is historically linked with the minting of silver coins (sterlings) from the Tower pound. Large payments were reckoned in “pounds of sterlings,” later shortened to “pounds sterling.”