# 2019 redefinition of the SI base units

In the redefinition, four of the seven SI base units – the kilogram, ampere, kelvin, and mole – were redefined by setting exact numerical values for the Planck constant (h), the elementary electric charge (e), the Boltzmann constant (k), and the Avogadro constant (N ), respectively. The kilogram remained defined by a physical prototype, leaving it the only artefact upon which the SI unit definitions depend. [17]

There is a precedent for changing the underlying principles behind the definition of the SI base units; the 11th CGPM (1960) defined the SI metre in terms of the wavelength of krypton-86 radiation, replacing the pre-SI metre bar and the 13th CGPM (1967) replaced the original definition of the second, which was based on Earth’s average rotation from 1750 to 1892,[18] with a definition based on the frequency of the radiation emitted between two hyperfine levels of the ground state of the caesium-133 atom.

Source: en.wikipedia.org