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The SI system of units defines seven SI base units: physical units defined by an operational definition. All other physical units can be derived from these base units: these are known as SI derived units. Derivation is by dimensional analysis. Use SI prefixes to abbreviate long numbers. The following are the base units from which all others are derived. They are dimensionally independent, with the exception of the metre and candela. The candela was formerly a fundamental unit but has been redefined in terms of the other SI base units. The metre also was a fundamental unit, but has since been redefined in terms of the second. They are still considered "base units" for historical reasons, but are in fact dependent on the other units for their definition.
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