SI base unit units are written in lowercase, except the symbols of those named after a person, which are written with an initial capital letter. For example, the "metre" (US English: "meter") has the symbol m, but the "kelvin" has symbol K, because it is named after Lord Kelvin and the "ampere" with symbol A is named after André-Marie Ampère. Several other units, such as the litre (US English: "liter"), are formally not part of the SI, but are accepted for use with SI. New definitions of the base units were approved on 16 November 2018, to take effect 20 May 2019. The
History of the metric system the Treaty of the Metre was extended to cover electrical units with the CGPM merging its work with that of the IEC. The twentieth century history of measurement is marked by five periods: the 1901 definition of the coherent MKS system; the intervening 50 years of coexistence of the MKS, cgs and common systems of measures; the 1948 "Practical system of units" prototype of the SI; the introduction of the SI in 1960; and the evolution of the SI in the latter half century. The need for an orthogonal electromagnetic unit to resolve the difficulties of defining such units in