This law was postulated in the 1930s, after the first and second laws of thermodynamics had been developed and named. It is called the zeroth law because it comes logically before the first and second laws (discussed in Thermodynamics). Suppose, for example, a cold metal block and a hot metal block are both placed on a metal plate at room temperature. Eventually the cold block and the plate will be in thermal equilibrium. In addition, the hot block and the plate will be in thermal equilibrium. By the zeroth law, we can conclude that the cold block and the hot block are also in thermal equilibrium. Similarly, two thermometers placed in thermal contact, one with one block and the second with the other block, will also be in thermal equilibrium and therefore read the same temperature. This establishes temperature as a meaningful concept.
Does the temperature of a body in internal thermal equilibrium depend on its size?
OpenStax College Physics 2e, 13.1 Temperature, exercise 1