The scale is based on 0° for the freezing point of water and 100° for the boiling point of water at 1 atm pressure. This scale is widely taught in schools today.
By international agreement the unit "degree Celsius" and the Celsius scale are currently defined by two different temperatures: absolute zero, and the triple point of VSMOW (specially purified water). This definition also precisely relates the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature with symbol K.
Absolute zero, the lowest temperature possible, is defined as being precisely 0 K and −273.15 °C. The temperature of the triple point of water is defined as precisely 273.16 K (0.01 °C; 32.02 °F) at 611.657 pascals (6.11657 mbar; 0.00603659 atm) pressure.[