NOTE 1 – In each coherent system of units, there is only one base unit for each base quantity. In the SI for example, the metre is the base unit of length. The centimetre and the kilometre are also units of length, but they are not base units in the SI. However, in the CGS systems, the centimetre is the base unit of length.

NOTE 2 – A base unit may also serve for a derived quantity of the same dimension. For example, rainfall, when defined as volume per area (areic volume), has the metre as a coherent derived unit in the SI. The ampere, base unit of electric current, is also the coherent derived unit of scalar magnetic potential.

NOTE 3 – For the quantity “number of entities”, the number one, symbol 1, can be regarded as a base unit in any system of units.