states: that if two of three separate systems can in separate experiments be shown to be in mutual thermal equilibrium with the third, then they will be found to be in mutual thermal equilibrium with each other. [P7.2]
sounds: so obvious as to make it unnecessary to glorify it with a name. In fact, as its name suggests, it was a bit of an afterthought. [P7.2]
but logically is: very important, because it leads directly to the concept of temperature and the setting up of temperature scales and the various procedures for temperature measurement. [P7.2]
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