Principal system of weights and measures used in a few nations, the only
major industrial one being the United States. It actually consists of two related systems: the U.S. Customary
System of units, used in the United States and dependencies, and the British Imperial System. The names of the
units and the relationships between them are generally the same in both systems, but the sizes of the units differ,
sometimes considerably.

Customary Units of Weights and Measures

Units of Weight

The pound (lb) is the basic unit of weight (which is proportional to mass). Within the English units of measurement
there are three different systems of weights. In the avoirdupois system, the most widely used of the three, the pound
is divided into 16 ounces (oz) and the ounce into 16 drams. The ton, used to measure large masses, is equal to 2,000 lb
(short ton) or 2,240 lb (long ton). In Great Britain the stone, equal to 14 lb, is also used. The troy system (named f
or Troyes, France, where it is said to have originated) is used only for precious metals. The troy pound is divided
into 12 ounces and the troy ounce into 20 pennyweights or 480 grains; the troy pound is thus 5,760 grains. The grain
is also a unit in the avoirdupois system, 1 avoirdupois pound being 7,000 grains, so that the troy pound is 5,760/7,000 of
an avoirdupois pound. Apothecaries’ weights are based on troy weights; in addition to the pound, ounce, and
grain, which are equal to the troy units of the same name, other units are the dram (1/8 oz) and the scruple (1/24 oz or 1/3 dram).
The basic unit of length is the yard (yd); fractions of the yard are the inch (1/36 yd) and the foot (1/3 yd), and commonly
used multiples are the rod (51/2 yd), the furlong (220 yd), and the mile (1,760 yd). The acre, equal to 4,840 square yards
or 160 square rods, is used for measuring land area.

Units of Liquid Measure

For liquid measure, or liquid capacity, the basic unit is the gallon, which is divided into 4 quarts, 8 pints, or 32 gills.
The U.S. gallon, or wine gallon, is 231 cubic inches (cu in.); the British imperial gallon is the volume of 10 lb of
pure water at 62°F and is equal to 277.42 cu in. The British units of liquid capacity are thus about 20% larger
than the corresponding American units. The U.S. fluid ounce is 1/16 of a U.S. pint; the British unit of the same name
is 1/20 of an imperial pint and is thus slightly smaller than the U.S. fluid ounce.

Units of Dry Measure

For dry measure, or dry capacity, the basic unit is the bushel, which is divided into 4 pecks, 32 dry quarts, or 64 dry pints.
The U.S. bushel, or Winchester bushel, is 2,150.42 cu in. and is about 3% smaller than the British imperial bushel of 2,219.36 cu
in., with a similar difference existing between U.S. and British subdivisions. The barrel is a unit for measuring the capacity
of larger quantities and has various legal definitions depending on the quantity being measured, the most common value being 105 dry quarts.

Differences between American and British Systems

Many American units of weights and measures are based on units in use in Great Britain before 1824, when the British Imperial
System was established. Since the Mendenhall Order of 1893, the U.S. yard and pound and all other units derived from them
have been defined in terms of the metric units of length and mass, the meter and the kilogram; thus, there is no longer
any direct relationship between American units and British units of the same name. In 1959 an international agreement was
reached among English-speaking nations to use the same metric equivalents for the yard and pound for purposes of science
and technology; these values are 1 yd=0.9144 meter (m) and 1 lb=0.45359237 kilogram (kg). In the United States, the older
definition of the yard as 3,600/3,937 m is still used for surveying, the corresponding foot (1,200/3,937 m) being known as the survey foot.
The English units of measurement have many drawbacks: the complexity of converting from one unit to another, the differences between
American and British units, the use of the same name for different units (e.g., ounce for both weight and liquid capacity, quart and
pint for both liquid and dry capacity), and the existence of three different systems of weights (avoirdupois, troy, and apothecaries’).
Because of these disadvantages and because of the wide use of the much simpler metric system in most other parts of the world, there
have been proposals to do away with the U.S. Customary System and replace it with the metric system.

source: The Columbia Encyclopedia, Sixth Edition. 2001.