In the middle of the 19th century, a coherent three-dimensional system of units was created using the base quantities length, mass and time. This was the absolute CGS system. The subsequent introduction of these absolute units into electrodynamics had many theoretical implications. Maxwell developed an elaborate metrological theory of two systems, the electrostatic and the electromagnetic system.

In 1901 Giovanni Giorgi succeeded in reformulating the existing theory of electromagnetic phenomena as a four-dimensional theory (Unità Razionali di Elettromagnetismo, Rational Units of Electromagnetism [1]).

The International Electrotechnical Commission (IEC) was founded in London in 1906 as an international forum where scientists and engineers could discuss all relevant questions.

In 1935 an IEC meeting in Scheveningen adopted a system comprising the three units metre, kilogram and second plus a fourth unit to be chosen later. This was called the Giorgi System. IEC TC 25 (Quantities and units, and their letter symbols) was established.

In 1950 the ampere was finally chosen as the fourth unit of the system.

In 1960 the General Conference on Weights and Measures adopted a resolution that the system based on metre, kilogram, second, ampere, kelvin and candela be given the name Système international d’unités (International System of Units), with the abbreviation SI. In 1971 the mole was added as a seventh base unit.

It is shown why many physicists were opposed to the Giorgi System. In fact some theoretical physicists, though not engineers, still occasionally use the CGS system today.

Among the present projects of IEC TC 25 in the field of units, the future publication ISO/IEC 80000: Quantities and units, is particularly noteworthy. It is intended to harmonize all the relevant IEC and ISO International Standards.