On 20 May 2019, the definition of the SI unit of length, the metre, was changed to follow the explicit-constant format, i.e. with the emphasis on the fact that the speed of light is a constant of nature and that by regarding it as such (and making use of the SI unit of time, the second) the unit of length may be obtained. This paper regards the history of the SI metre before examining in more detail the revised definition and the methods of realising the metre in practice according to the Mise en Pratique document approved by the International Committee on Weights and Measures (CIPM).
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