United States customary units are a system of measurements commonly used in the United States. The U.S. customary system developed from English units which were in use in the British Empire before American independence. Consequently most U.S. units are virtually identical to the British imperial units. However, the British system was overhauled in 1824, changing the definitions of some units used there, so several differences exist between the two systems.
The majority of U.S. customary units were redefined in terms of the meter and the kilogram with the Mendenhall Order of 1893, and in practice, for many years before. These definitions were refined by the international yard and pound agreement of 1959. The U.S. primarily uses customary units in its commercial activities, while science, medicine, government, and many sectors of industry use metric units. The SI metric system, or International System of Units is preferred for many uses by NIST
Many different units of length have been used around the world. The main units in modern use are U.S. customary units in the United States and the Metric system elsewhere. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units.
The base unit in the International System of Units (SI) is the metre, defined as "the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second." It is approximately equal to 1.0936 yards. Other units are derived from the metre by adding prefixes from the table below:
The system of imperial units or the imperial system (also known as British Imperial) is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced. The system came into official use across the British Empire. By the late 20th century, most nations of the former empire had officially adopted the metric system as their main system of measurement, but some Imperial units are still used in the United Kingdom and Canada.