♠  The Definition of the Kelvin and the International Temperature Scale
    The kelvin (K), unit of thermodynamic temperature, is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The International Temperature Scale (ITS) is a temperature scale which is designed to be consistent with thermodynamic temperatures measured on the basis of this definition. ITS is calibrated by using temperature fixed-points (defining fixed-points), which are obtained when specific substances are in phase equilibrium, and several stable thermometers. The first International Temperature Scale was established in 1927. After a series of subsequent extensions of temperature range and precision improvements, the International Temperature Scale of 1990 (ITS-1990) was adopted and is still in use today. Numerical values in the unit of Celsius temperature, symbol °C (degree Celsius), are produced by those in kelvins minus 273.15.

Defining fixed points of the International Temperature Scale of 1990 (ITS-90) and interpolating thermometers