Before 2019, the triple point of water was used to define the kelvin, the base unit of thermodynamic temperature in the International System of Units (SI). The kelvin was defined so that the triple point of water is exactly 273.16 K, but that changed with the 2019 redefinition of SI base units, where the kelvin was redefined so that the Boltzmann constant is exactly 1.380649×10 J⋅K, and the triple point of water became an experimentally measured constant.