Fundamental and Derived Units

It is found that this principle, when fully carried out, leaves us free to choose arbitrarily three units, which are therefore called fundamental units, and that most of the other units employed in physical measurement can be defined with reference to the fundamental units by the consideration that the factor k in the equations connecting them shall be equal to unity. Units obtained in this way are called derived units, and all the derived units belong to an absolute system based on the three fundamental units.