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Customary international law are those aspects of international law that derive from custom. Along with general principles of law and treaties, custom is considered by the International Court of Justice, jurists, the United Nations, and its member states to be among the primary sources of international law.

For example, laws of war were long a matter of customary law before they were codified in the Hague Conventions of 1899 and 1907, Geneva Conventions, and other treaties.

The vast majority of the world's governments accept in principle the existence of customary international law, although there are many differing opinions as to what rules are contained in it.

Recognition of customary international law
There are several different kinds of customary international laws recognized by states. Some customary international laws rise to the level of jus cogens through acceptance by the international community as non-derogable rights, while other customary international law may simply be followed by a small group of states. States are typically bound by customary international law regardless of whether the states have codified these laws domestically or through treaties.

Jus cogens
See jus cogens.

A peremptory norm (also called jus cogens, Latin for "compelling law") is a fundamental principle of international law which is accepted by the international community of states as a norm from which no derogation is ever permitted. These norms rooted from Natural Law principles,, and any laws conflicting with it should be considered null and void. Examples include various international crimes; a state which carries out or permits slavery, torture, genocide, war of aggression, or crimes against humanity is always violating customary international law.

Jus cogens and customary international law are not interchangeable. All jus cogens are customary international law through their adoption by states, but not all customary international laws rise to the level of peremptory norms. States can deviate from customary international law by enacting treaties and conflicting laws, but jus cogens are non-derogable.

Codification of international customary law
Some international customary laws have been codified through treaties and domestic laws, while others are recognized only as customary law.

Silence as consent
Generally, sovereign nations must consent in order to be bound by a particular treaty or legal norm. However, international customary laws are norms that have become pervasive enough in the international that countries need not consent in order to be bound. In these cases, all that is needed is that the state has not objected to the law. However, states that object to customary international law before these laws may not be bound by them unless these laws are deemed to be jus cogens.

The International Court of Justice
The Statute of the International Court of Justice acknowledges the existence of customary international law in Article 38(1)(b), incorporated into the United Nations Charter by Article 92: "The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply...international custom, as evidence of a general practice accepted as law."

Customary international law "... consists of rules of law derived from the consistent conduct of States acting out of the belief that the law required them to act that way." It follows that customary international law can be discerned by a "widespread repetition by States of similar international acts over time (State practice); Acts must occur out of sense of obligation (opinio juris); Acts must be taken by a significant number of States and not be rejected by a significant number of States." A marker of customary international law is consensus among states exhibited both by widespread conduct and a discernible sense of obligation.

The International Court of Justice held in Nicaragua v. United States that the elements of an international customary law would be Opinio Juris (Past Judge Decisions or works of the most highly qualified publicists) which is then proven by existing state practices.

Bilateral versus multilateral customary international law
The recognition of different customary laws can range from simple bilateral recognition of customary laws to world-wide multilateral recognition. Regional customs can become customary international law in their respective regions, but do not become customary international law for nations outside the region.

Other customary international laws
Other examples accepted or claimed as customary international law include the principle of non-refoulement and immunity of visiting foreign heads of state.