There are two requirements for a custom to be accepted as law: (i) there should be sufficiently uniform general practice accepted as law, and (ii) the belief that such a practice is obligatory. Consequently, for a practice or usage to become a customary rule of international law, the following two factors are essential:

• A material fact: in similar circumstances States act similarly, in other words, usage has been constantly and uniformly practiced by States. 

• A psychological element: (opinio juris sive necessitatis) (an opinion of law or necessity) - the feeling on the part of States that in acting as they act they are fulfilling a legal obligation.

The two are often referred to as the objective (material) and subjective elements of custom respectively