Custom in law is the established pattern 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." Customary law exists where:
- a certain legal practice is observed and
- the relevant actors consider it to be law (opinio juris).
Related is the idea of prescription; a right enjoyed through long custom rather than positive law.
The modern codification of civil law developed from the tradition of medieval customaries, collections of customary law that developed in particular communities, slowly gathered, and later written down by local jurists. Customaries acquired the force of law when they became the undisputed rule by which certain rights, entitlements, and obligations were regulated between members of a community. The Coutume de Paris - a major collection of the laws of the city of Paris - is an example.
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.
Customary law is a recognized source of law within jurisdictions of the civil law tradition, 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.[dubious – discuss] Customary law is also used in some Third World countries, such as those in Africa, usually used alongside common or civil law. For example in Ethiopia, despite the adoption of legal codes based on civil law in the 1950s according to Dolores Donovan and Getachew Assefa there are more than 60 systems of customary law currently in force, "some of them operating quite independently of the formal state legal system." They offer two reasons for the relative autonomy of these customary law systems: one is that the Ethiopian government lacks sufficient resources to enforce its legal system to every corner of Ethiopia; the other is that the Ethiopian government has made a commitment to preserve these customary systems within its boundaries.
In 1995, President of Kyrgyzstan Askar Akaev announced a decree to revitalize the aqsaqal courts of village elders. The courts would have jurisdiction over property, torts and family law. The aqsaqal courts were eventually included under Article 92 of the Kyrgyz constitution. As of 2006, there were approximately 1,000 aqsaqal courts throughout Kyrgyzstan, including in the capital of Bishkek. Akaev linked the development of these courts to the rekindling of Kyrgyz national identity. In a 2005 speech, he connected the courts back to the country's nomadic past and extolled how the courts expressed the Kyrgyz ability of self-governance. Similar aqsaqal courts exist, with varying levels of legal formality, in other countries of Central Asia.
The Somali people in the Horn of Africa follow a customary law system referred to as Xeer. It survives to a significant degree everywhere, including the Somali communities in the Ogaden. Economist Peter Leeson attributes the increase in economic activity since the fall of the Siad Barre administration to the security in life, liberty and property provided by Xeer in large parts of Somalia. The late Dutch attorney Michael van Notten also draws upon his experience as a legal expert in his comprehensive study on Xeer, The Law of the Somalis: A Stable Foundation for Economic Development in the Horn of Africa (2005).
In India many customs are accepted by law.For. eg Marriage ceremonies in Hindus are accepted in Indian marriage Act.
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.