This Brochure is concerned with presenting the information necessary to define and use the International System of Units, universally known as the SI (from the French Système International d'Unités). The SI was established by and is defined by the General Conference on Weights and Measures, the CGPM, as described in the Historical note in Section 1.8*.
The system of quantities, including the equations relating the quantities, to be used with the SI, is in fact just the quantities and equations of physics that are familiar to all scientists, technologists, and engineers. They are listed in many textbooks and in many references, but any such list can only be a selection of the possible quantities and equations, which is without limit. Many of the quantities, their recommended names and symbols, and the equations relating them, are listed in the International Standards ISO 31 and IEC 60027 produced by Technical Committee 12 of the International Organization for Standardization, ISO/TC 12, and by Technical Committee 25 of the International Electrotechnical Commission, IEC/TC 25. The ISO 31 and IEC 60027 Standards are at present being revised by the two standardization organizations in collaboration. The revised harmonized standard will be known as ISO/IEC 80000, Quantities and Units, in which it is proposed that the quantities and equations used with the SI will be known as the International System of Quantities.
The base quantities used in the SI are length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. The base quantities are by convention assumed to be independent. The corresponding base units of the SI were chosen by the CGPM to be the metre, the kilogram, the second, the ampere, the kelvin, the mole, and the candela. The definitions of these base units are presented in Section 2.1.1 in the following chapter. The derived units of the SI are then formed as products of powers of the base units, according to the algebraic relations that define the corresponding derived quantities in terms of the base quantities, see Section 1.4.
On rare occasions a choice may arise between different forms of the relations between the quantities. An important example occurs in defining the electromagnetic quantities. In this case the rationalized fourquantity electromagnetic equations used with the SI are based on length, mass, time, and electric current. In these equations, the electric constant _{0} (the permittivity of vacuum) and the magnetic constant _{}_{0} (the permeability of vacuum) have dimensions and values such that _{0}_{}_{0} = 1/c_{0}^{2}, where c_{0} is the speed of light in vacuum. The Coulomb law of electrostatic force between two particles with charges q_{1} and q_{2} separated by a distance r is written**
F = 
q_{1}q_{2} r 

4_{0} r^{3} 
and the corresponding equation for the magnetic force between two thin wire elements carrying electric currents, i_{1}dl_{1} and i_{2}dl_{2}, is written
d^{2}F = 
_{}_{0} 
i_{1}dl_{1} x (i_{2}dl_{2} x r) 


4 
r^{3} 
where d^{2}F is the double differential of the force F. These equations, on which the SI is based, are different from those used in the CGSESU, CGSEMU, and CGSGaussian systems, where _{0} and _{}_{0} are dimensionless quantities, chosen to be equal to one, and where the rationalizing factors of 4 are omitted.
* 
Acronyms used in this Brochure are listed with their meaning here. 
** 
Symbols in bold print are used to denote vectors. 


The name Système International d'Unités, and the abbreviation SI, were established by the 11th CGPM in 1960.
Examples of the equations relating quantities used in the SI are the Newtonian inertial equation relating force, F, to mass, m, and acceleration, a,
for a particle: F = m a, and the equation giving the kinetic energy, T, of a particle moving with velocity, :
T = m^{2}/2.
