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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." Related is the idea of prescription; a right enjoyed through long custom rather than positive law.
Customary law (also, consuetudinary or unofficial law) exists where:
- a certain legal practice is observed and
- the relevant actors consider it to be law (opinio juris).
Most customary laws deal with standards of community that have been long-established in a given locale. However the term can also apply to areas of international law where certain standards have been nearly universal in their acceptance as correct bases of action - in example, laws against piracy or slavery (see hostis humani generis). In many, though not all instances, customary laws will have supportive court rulings and case law that has evolved over time to give additional weight to their rule as law and also to demonstrate the trajectory of evolution (if any) in the interpretation of such law by relevant courts.
Customary law and codification 
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.
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. However, many treaties are attempts to codify pre-existing customary law.
Customary law within contemporary legal systems 
Customary law is a recognized source of law within jurisdictions of the civil law tradition, where it may be subordinate 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." On the other hand, in many countries around the world, one or more types of customary law continue to exist side by side with official law, a condition referred to as legal pluralism (see List of national legal systems).
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 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).
Custom in torts 
|This section may be confusing or unclear to readers. (November 2008)|
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.
Custom law in India 
Customary legal systems 
- Adat (Malays of Nusantara)
- Anglo-Saxon law (England)
- Aqsaqal (Central Asia)
- Coutume (France)
- Customary law (Australia)
- Early Germanic law
- Early Irish law (Ireland)
- Laws of the Brets and Scots (Scotland)
- Medieval Scandinavian laws
- Pashtunwali and Jirga (Pashtuns of Pakistan and Afghanistan)
- Smriti and Ācāra (India)
- Customary law (South Africa)
- Urf (Arab world/Islamic law)
- Welsh Law (Wales)
- Xeer (Somalia)
- Usos y costumbres (various regions of Latin America)
See also 
- Civil law (legal system)
- Common law
- Customary international humanitarian law
- Journal of Legal Pluralism and Unofficial Law
- Legal dualism
- Legal pluralism
- Jus gentium (law of nations)
- Rule according to higher law
- Rule of law
- Time immemorial, has a specific meaning in English Law
- In R. v Secretary of State For Foreign and Commonwealth Affairs,  2 All E.R. 118, Lord Denning said "These customary laws are not written down. They are handed down by tradition from one generation to another. Yet beyond doubt they are well established and have the force of law within the community."
- John Henry Merryman, The Civil Law Tradition, p. 23 (2d Ed. 1985)
- "Constitution Act, 1982, s. 35(1)". Retrieved 29 July 2008.
- Slattery, Brian. Generic and Specific Aboriginal Rights. p. 6. Retrieved 21 August 2008. and Foster, Hamar; Heather Raven and Jeremy Webber (eds.) (2007). Let Right Be Done: Aboriginal title, the Calder Case, and the Future of Indigenous Rights. 18 No. 7 (July, 2008). Vancouver: UBC Press. pp. 574–578. Retrieved 11 September 2010.
- "JuryGlobe". University of Ottawa. Retrieved 11 September 2010.
- Dolores A. Donovan and Getachew Assefa, "Homicide in Ethiopia: Human Rights, Federalism, and Legal Pluralism," American Journal of Comparative Law, 51 (2003), p. 505
- Judith Beyer, Kyrgyz Aksakal Courts: Pluralistic Accounts of History, 53 J. OF L. PLURALISM 144 (2006)
- Former President Akaev, quoted in Beyer, Kyrgyz Aksakal Courts
- Spencer Heath MacCallum (12 September 2007). "The Rule of Law without the State". Ludwig von Mises Institute. Retrieved 11 September 2010.
- "Grassroots Conflict Assessment Of the Somali Region, Ethiopia". CHF International. August 2006. Retrieved 11 September 2010.
- "Better off stateless". Retrieved 11 September 2010.
- Van Notten, Michael. 2005. The Law of the Somalis: A Stable Foundation for Economic and Social Development in the Horn of Africa, Trenton NJ: Red Sea Press.