For many years, the international standard for defining all units of length was the meter as defined by the distance between two scratches on a platinum-iridium bar.
The bar is stored in a vault at the International Bureau of Weights and Measures at Sevres, near Paris.
In 1960, the basis of the standard was changed to the wavelength of orange light emitted by the gas krypton 86.
In 1984, the General Conference on Weights and Measures in Paris switched once more, to a standard based on the speed of light.
Under this system, one meter is defined as the distance traveled by light through a vacuum in 1/299,792,458th of a second.
That definition will remain in use for the foreseeable future, according to the National Institute of Standards and Technology, formerly the National Bureau of Standards.