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
Unit of measurement extent based on the dimensions of the human body. As a result, units of measure could vary not only from location to location, but from person to person. Metric systems of units have evolved since the adoption of the original metric system in France in 1791. The current international standard metric system is the International System of Units (abbreviated to SI). An important feature of modern systems is standardization. Each unit has a universally recognized size. Both the imperial units and US customary units derive from earlier English units. Imperial units were mostly used in the British Commonwealth and the