the two systems of linear measure in use today are
Imperial and US customary measurement systems Most of the units of measure have been adopted in one way or another since the Norman Conquest (1066). The units of linear measure have changed the least – the yard (which replaced the ell) and the chain were measures derived in England. The foot used by craftsman supplanted the foot used in agriculture which was reduced in size by a factor of  10⁄11 to bring it into alignment. This resulted in there being  16 1⁄2 feet in a rod, pole or perch instead of 15 agricultural feet. The furlong and the acre, once it became a unit of the size of a piece of land rather than its value, remained relatively unchanged. In the last thousand years, three principal pound were used in England. The troy pound (5760 grains) was used for precious metals, the apothecaries' pound, (also 5760 grains) was used by pharmacists and the avoirdupois pound (7000 grains) was used for general purposes. The apothecaries and troy pounds are divided into 12 ounces (of 480 grains) while the avoirdupois pound has 16 ounces (of 437.5 grains). The unit of volume, the gallon, has different values in the United States and in the United Kingdom – the US fluid gallon being about 0.83 imperial gallons and the US dry gallon being about 0.97 imperial gallons.