• Lower fixed point of a thermometer

The lower fixed point, also known as the ice point, is defined as the temperature at which pure water freezes (melting ice) at standard atmospheric pressure.

This point is marked at 0°C.

• Upper fixed point of a thermometer

The upper fixed point, referred to as the steam point, is defined as the temperature at which pure water boils (steam) at standard atmospheric pressure.

This point is marked at 100°C.