Until 1960, the international length standard was defined by two marks inscribed on a bar of platinum-iridium. In 1960, the definition of the standard changed.
When energy is imparted to an atom, photons are emitted at various wavelengths producing a well-defined and repeatable spectrum for that element. The meter was now defined by the General Conference on Weights and Measures as 1 650 763.73 wavelengths of the orange-red line in the spectrum of radiation from electrically excited atoms of krypton-86.
The meter as an artifact had been replaced by a definition of the meter in terms of a constant of nature. Realization of the standard for purposes of making comparisons between it and secondary standards was now universally achievable.
Previously, only the International Bureau of Weights and Measures (BIPM) had realized the standard - in the act of defining it. In 1960, the wavelength standard became universally reproducible and indestructible.