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.
2019 redefinition of SI base units effect of redefining the SI base units, though the definitions of the SI derived units in terms of the base units remain the same. Following the CCU proposal, the text of the definitions of all the base units was either refined or rewritten changing the emphasis from explicit-unit to explicit-constant type definitions. Explicit-unit type definitions define a unit in terms of a specific example of that unit – for example in 1324 Edward II defined the inch as being the length of three barleycorns and since 1889 the kilogram has been defined as being the mass of the International Prototype