- For a topical guide to this subject, see Outline of the metric system.
The MKS system of units is a physical system of units that expresses any given measurement using fundamental units of the metre, kilogramme, and/or second (MKS). In 1901, Giovanni Giorgi proposed to the Associazione elettrotecnica italiana (AEI) that this system, extended with a fourth unit to be taken from the units of electromagnetism, be used as an international system.
Historically the MKS system of units succeeded the centimetre–gram–second system of units and laid the blueprint for the International System of Units, which now serves as the international standard. Therefore the exact composition of the MKS system is a historical issue. As a matter of historical record the MKS system incorporated fundamental units other than the metre, kilogram, and second in addition to derived units. An incomplete list of the fundamental and derived units appears below. Since the MKS system of units never had a governing body to rule on a standard definition, the list of units depended on different conventions at different times.
- Cycle. (This dimensionless quantity became synonymous with the term "cycle per second" as an abbreviation. This circumstance confused the exact definition of the term cycle. Therefore the phrase "cycle per metre" became ill-defined. The cycle did not become an SI unit.)
- Cycle per second.
- Cycle per metre. (This measure of wavenumber became ill-defined due to the abbreviation of "cycle per second" as "cycle".)