SI derived unit m), the SI derived unit of density. The names of SI derived units, when written in full, are in lowercase. However, the symbols for units named after persons are written with an uppercase initial letter. For example, the symbol for hertz is "Hz"; but the symbol for metre is "m". The International System of Units assigns special names to 22 derived units, which includes two dimensionless derived units, the radian (rad) and the steradian (sr). Some other units such as the hour, litre, tonne, bar and electronvolt are not SI units, but are widely used in conjunction with SI units.
Unit of length in vacuum during a time interval of seconds." It is approximately equal to . Other units are derived from the metre by adding prefixes from the table below: For example, a kilometre is . In the Centimetre–gram–second system of units, the basic unit of length is the centimetre, or of a metre. Other non-SI units are derived from decimal multiples of the metre. The basic unit of length in the Imperial and U.S. customary systems is the yard, defined as exactly by international treaty in 1959. Common Imperial units and U.S. customary units of length include: In addition, the following