In law, custom can be described as the established patterns of behavior that can be objectively verified within a particular social setting. A claim can be carried out in defense of "what has always been done and accepted by law." Generally, customary law exists where:
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. However, many treaties are attempts to codify pre-existing customary law. With this said customary international law is often question as to its validity
Customary law is a recognized, but inferior, source of law within jurisdictions of the civil law tradition. It is strictly inferior to both statutes and regulations. In addressing custom as a source of law within the civil law tradition, John Henry Merryman notes that, though the attention it is given in scholarly works is great, its importance is "slight and decreasing.
In Canada, customary aboriginal law has a constitutional foundation and for this reason has increasing influence.
In the Scandinavian countries customary law continues to exist and has great influence. Customary law is also used in some Third World countries, such as in Africa, usually used alongside common or civil law.
Custom is used in tort law to help determine negligence. Following or disregarding a custom is not determinative of negligence, but instead is an indication of possible best practices or alternatives to a particular action. The case R v. Boomsdale defines this principle with the courts ruling that Mr Boomsdale customary practice was not sufficient to be deemed an act of negligence
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