Water's freezing point, commonly known as 0 °C, varies slightly based on the isotopes of hydrogen and oxygen present. These isotopes, like deuterium and oxygen-18, are heavier and cause water to freeze at slightly higher temperatures. Natural isotope ratios on Earth are not constant, leading to the need for a standardized reference.

In 1961, Harmon Craig proposed "Standard Mean Ocean Water" (SMOW). After conflicting standards emerged, the International Atomic Energy Agency adopted Craig's proposal, creating an official international standard known as Vienna Standard Mean Ocean Water (VSMOW). A second standard, SLAP, was created to represent rainwater.

VSMOW is the ultimate standard for metrology, used to calibrate precise instruments. A small ampoule of VSMOW costs $159, equating to approximately $120,000 per gallon. This expensive water is crucial for calibrating triple point cells, which maintain a precise equilibrium of ice, liquid water, and water vapor at 0.01 °C, a value historically used to define the Kelvin scale.