NOTE 1 In each coherent system of units, there is only one base unit for each base quantity.
EXAMPLE In the SI, the metre is the base unit of length. In the CGS systems, the centimetre is the base unit of length.
EXAMPLE Rainfall, when defined as areic volume (volume per area), has the metre as a coherent derived unit in the SI.
NOTE 3 For number of entities, the number one, symbol 1, can be regarded as a base unit in any system of units.