The Zeroth Law of Thermodynamics

0^th Law of Thermodynamics: Two objects in thermal equilibrium with a third object re also in equilibrium with each other.

The 0^th Law is called the 0^th Law and not the 4^th because it is more fundamental than the 1^st law but was discovered after the other three. 

The idea is like having three interconnected beakers of water such that the water levels in all three are the same height. Furthermore because the beakers are interconnected any change in one gets balanced out so all three end up with the same level.

The zeroth law of Thermodynamics is a simple concept and which why it is the most fundamental of the Laws of Thermodynamics.