A system of units is said to be coherent if all of its units are either base units, or are derived from the base units without using any numerical factors other than 1. The International System of Units (SI) is a coherent system. The American customary system of units is not coherent, because, for example, the factor 550 is used in defining the unit of power, horsepower (550 foot-pounds per second).
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Last revised: 16 October 2003.