In law, custom, or customary law consists of established patterns of behaviour that can be objectively verified within a particular social setting. Generally, customary law exists where

• a certain legal practice is observed; and
• the relevant actors consider it to be law (opinio iuris).

Customary law and codification

The modern codification of civil law developed out of the customs, or coutumes of the middle ages, expressions of law that developed in particular communities and slowly collected and written down by local jurists. Such customs acquired the force of law when they became the undisputed rule by which certain entitlements (rights) or obligations were regulated between members of a community.

International law

In international law, customary law refers to the Law of Nations or the legal norms that have developed through the customary exchanges between states over time, whether based on diplomacy or aggression. Essentially, legal obligations are believed to arise between states to carry out their affairs consistently with past accepted conduct. These customs can also change based on the acceptance or rejection by states of particular acts. Some principles of customary law have achieved the force of peremptory norms, which cannot be violated or altered except by a norm of comparable strength. These norms are said to gain their strength from universal acceptance, such as the prohibitions against genocide and slavery. Customary international law can be distinguished from treaty law, which consists of explicit agreements between nations to assume obligations. Many treaties, however, are attempts to codify pre-existing customary law.

Customary law within contemporary legal systems

Customary law may also be relevant within jurisdictions following another legal tradition such as civil law in fields or subfields of law where no legislative enactment exists. For example, in Austria, scholars of private law often claim that customary law continues to exist, whereas public law scholars dispute this claim. In any case, it is hard to find any practically relevant examples.

Customary international law

In addition to treaties and other expressed or ratified agreements that create international law, the International Court of Justice, legal scholars, jurists, the United Nations and its member states consider customary international law, coupled with General principles of law, to be primary sources of international law. The vast majority of the worlds governments (including the United States) accept in principle the existence of customary international law, although there are many differing opinions as to what rules are contained in it.

The UN charter acknowledges the existence of customary international law (article 38(1)(b) of the Statute, incorporated into the Charter by article 92 thereof): "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." (Rosenne, Practice and Methods of International Law, p. 55). 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."

Even extreme conservatives have acknowledged the existence of customary international law. For example, Doug Bandow of the Cato Institute testifed to the US Senate 8 April 2004 >more</a> »</p> in regards to the UNCLOS that:

The treaty's authorization of 200-mile exclusive economic zones (EEZs) merely reflects what has become customary international law.

[...]

International law analyst Gary Knight even argues that "the difficulty of establishing our legal right to EEZ navigation and submerged straits passage would be no more difficult under an existing customary international law argument than under the convoluted text of the proposed UNCLOS." In short, there is only modest theoretical advantage in this area for which to trade away the mining provisions.

Amnesty International writes that:

Customary international law results from a general and consistent practice of states followed out of a sense of legal obligation, so much so that it becomes custom. As such, it is not necessary for a country to sign a treaty for customary international law to apply.

In other words, customary international law must be derived from a clear consensus among states, as exhibited both by widespread conduct and a discernible sense of obligation.

Customary international law can therefore not be declared by a majority of States for their own purposes; it can be discerned only through actual widespread practice. For example, laws of war were long a matter of customary law before they were codified in the Geneva Conventions and other treaties.

A particular category of customary international law, Jus Cogens refers to a principle of international law so fundamental that no state may opt out by way of treaty or passage of domestic law. Examples of this is the act of genocide and crimes against humanity.