The metre was originally defined in 1791 as one ten-millionth of the distance from the equator to the North Pole along a great circle.
The International Bureau of Weights and Measures maintains the techniques for realisation of the base units in the International System of Units (SI), all seven of which are defined in terms of natural physical constants, rather than human artefacts such as the standard kilogram.
[2][3] Following the 2019 revision of the SI all fundamental units of metrology are now defined in terms of natural physical constants, rather than human artefacts.
This is a detailed set of technical instructions for the construction of a device that will produce a practical realization of each unit.
Historically two distinct approaches were used – one was to use a naturally occurring phenomenon such as a particular seed or part of the human body, the other was to use a standard length that was held by a community leader.