<P> The imperial and US customary systems of measurement are two closely inter-related systems of measurement both derived from earlier English system of measurement units which can be traced back to Ancient Roman units of measurement, and Carolingian and Saxon units of measure . </P> <P> US Customary units, developed and used in the United States after the American Revolution, are based on a subset of the English units used in the Thirteen Colonies, while the Imperial system of units was developed and used beginning in 1826 in the United Kingdom and subsequently used in the rest of the Commonwealth . US Customary units are the predominant system of units in the United States, but in all Commonwealth countries the metric system has, to varying degrees, replaced the imperial system . </P> <P> 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 ⁄ to bring it into alignment . This resulted in there being 16 ⁄ 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 . </P> <P> Both systems of measure were widely used in mechanical engineering, though not in electrical engineering . Some units of measure such as the horsepower or the British thermal unit (BTU) have special names but by and large unit names are generated from their constituent components--for example, pounds per square inch . In contrast, the metric system has a special name for pressure--the pascal . </P>

The two systems of linear measure in use today are
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