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The International System of Units (French: Système international d'unités, 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 began 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 24th and 25th General Conferences on Weights and Measures (CGPM) in 2011 and 2014, for example, discussed a proposal to change the definition of the kilogram, linking it to an invariant of nature rather than to the mass of a material artefact, thereby ensuring long-term stability. 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 on the rules for 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. 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.

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