Zeroth Law of Thermodynamics

The zeroth law of thermodynamics states that if two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other, the principle that makes temperature a well defined, transitive property in the first place and the basis on which a thermometer can be trusted to compare two objects it never touches at the same time. Despite underlying the first three laws logically, it was identified and named only after them: Ralph H. Fowler coined the term zeroth law of thermodynamics in the 1930s, and Fowler and Edward A. Guggenheim published the formal statement in 1936. Its number reflects that logical priority rather than the order of discovery, placed before the first law without renumbering the three laws already in wide use.