One of the sources of international law listed in article 38(1) is ‘international custom, as evidence of a general practice accepted as law’. From the definition follows, that there are two main requirements for customary law: objective requirement- ‘general practice’ (mainly, that states are practicing the law and it is referring to what states do) and subjective one- ‘accepted as law’ (opinion iuris of states, what states think they should do). Evidence of customary law can be found in treaties, writings of international lawyers and judgements of national and international tribunals.
During Nicaragua case, ICJ excluded jurisdiction with regard to the relevant treaty law and proceeded with customary international law, the content of which was the same as that laid down in Charter. One of the characteristics of customary law is apparent from the case- is that, treaty law and customary law can exist side by side.
In order for a state to rely on customary law there is a need to prove that relevant practice exists. Creation of customary law can be prevented by major inconsistencies, for instance, as in Asylum case, where question of whether Colombia had the right to grant asylum or handed him (leader of rebellion in Peru) over to Peruvian authorities. But the creation can’t be prevented because of minor inconsistencies, as from the Fisheries case, the Court could make a decision and upheld Norwegian method of delimitation of the territorial sea. From the Fisheries case, we can state that -although the practice of law involves small number of states and can last for a short amount of time, customary law can be created, when there is no practice against alleged rule of customary law.
From consideration the law of sea, where practice of sea powers and maritime nations have greater significance than practice of land-locked states, becomes known that there is no guidance on number of states for the practice, rather practice should reflect acceptance of it among affected states. States can be bound by general practice of other states, even if they wish against it, but not protesting against the emergence of the rule, for instance, in Legality of Nuclear Weapons case, statement emphasized by Judge Schwebel in his Dissenting Opinion was saying that, nuclear practice has been recognized and accepted by international community.
State practice is consisting of both- what states say and what states do. For instance, in Fisheries Case and Fisheries Jurisdiction cases, in the first case customary law (regarding claims made to areas of the sea by a state) couldn’t be created because, unless such claims were enforced against foreign ships, but in the later cases judges stated the existence of customary rules, from such claims, without considering whether they had been enforced.
In conclusion, customary international law has number of sources and there are many discussions over application of the law, but there are only two requirements, which are upholding the provision of customary international law.