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The seven base units in the International System of Units

The International System of Units (French: Système International d'Unités{{#invoke:Category handler|main}}, SI) is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units. It defines twenty-two named units, and includes many more unnamed coherent derived units. The system also establishes a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.

The system was published in 1960 as the result of an initiative that started in 1948. It is based on the metre-kilogram-second system of units (MKS) rather than any variant of the centimetre–gram–second system (CGS). SI is intended to be an evolving system, so prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves. The 25th General Conference on Weights and Measures (CGPM) in 2014, for example, discussed a proposal to change the definition of the kilogram.<ref name="25th CGPM">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The motivation for the development of the SI was the diversity of units that had sprung up within the CGS systems and the lack of coordination between the various disciplines that used them. The CGPM, which was established by the Metre Convention of 1875, brought together many international organisations to not only agree on the definitions and standards of the new system but also agree rules on writing and presenting measurements in a standardised manner around the world.

The International System of Units has been adopted by most developed countries, however, the adoption has not been universal in all English-speaking countries. While metrication in the United States is consistent in science, medicine, government, and various fields of technology and engineering, common measurements are mostly performed in United States customary units, although these have officially been defined in terms of SI units. The United Kingdom has officially adopted a policy of partial metrication, with no intention{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }} of replacing imperial units immediately. Canada has adopted the SI for most governmental, medical and scientific purposes and for such varied uses as grocery weights, weather reports, traffic signs and gasoline sales, but imperial units are still legally permitted and remain in common use throughout many sectors of Canadian society, particularly in the building trade and the railway sector.


International System of Units sections
Intro  History  SI Brochure and conversion factors  [[International_System_of_Units?section={{safesubst:#invoke:anchor|main}}_Units_and_prefixes|{{safesubst:#invoke:anchor|main}} Units and prefixes]]  [[International_System_of_Units?section=Writing_unit_symbols_and_the_values_of_quantities_{{safesubst:#invoke:anchor|main}}|Writing unit symbols and the values of quantities {{safesubst:#invoke:anchor|main}}]]  Realisation of units  Post-1960 changes  Global adoption  Redefinition of units  See also   Notes   References  Further reading  External links  

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