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Customary law, in its simplest form, is a body of rules, unofficial and generally unwritten, established through cultural or societal norms. When a society considers certain behaviors or practices so common as to be required by law, whether officially recorded or not, those beliefs establish the basis for customary law. Such practices apply to small tribal societies, civilized nations, and even international law. Customary law applies in international law when the conduct of countries becomes so consistent that it is universally believed to be codified as actual law.
In medieval times, civil law or customary law was the norm, serving as the primary form of legal statute. Rules were understood to be binding as law, without regard to their written status, since most laws, save royal decrees, were seldom written. Certain practices became law purely by repetition in regards to their application and lack of dispute. If a particular rule was regularly applied and behaviors regulated without dispute, citizens behaved in accordance with the rule as if required by legal obligation. Late in the medieval times, customary laws in many countries were gathered into written form, providing the basis for modern codification of official laws.
Customary law still provides the basis for new laws in many modern societies. For example, in the United States, the United Kingdom, and other countries, customary laws take the form of common law. Modern legal issues, such as the application of commerce laws to the Internet in the 1990s, started as customary laws. Judges ruled, where applicable, as if various disputes or behaviors occurred in similar real world situations rather than in a virtual world, since no official law was directly applicable. As rules became established with routine practices and limited dispute legislators enacted codified laws to mirror customary law, thus allowing for universal enforceability.
As applied to international law, customary law is also not an antiquated practice. In fact, laws based on accepted conduct are the basis for much of today’s modern international laws. Also known as the Law of Nations, such international civil law must meet specific criteria to be considered customary law. Criteria include three components: widespread recurrence, a sense of obligation, and little dispute. In short, these three criteria mean that numerous countries must engage in similar conduct, over a long period, out of a sense of perceived obligation, with little dispute over such conduct by other nations.
The same three criteria are often applied in terms of establishing such laws, whether local or international. A rule regarding accepted conduct must first be established and brought into common practice. Individuals, businesses, or countries must adhere to such rules out of a perceived legal obligation, believing the rule to be so routinely applied as to have the power of law. Few objections or disputes over the validity of the rule are permitted in order for the rule to become customary law.