customary international law
What might be called the “common law” of the law of nations. It consists of a vast body of detailed rules that, until the dawn of the 20th century, constituted the chief body of international law. Many of these rules, such as those relating to maritime law, had their origin in the practice of a single state, the UK, which was able to impose its will until the rules came to be accepted by other states. Other rules, notably those relating to commercial transaction, had their origin in the voluntary practice of a small number of states and, being found useful and convenient, were gradually adopted by other states until the established practice became a binding rule. Customary law was thus almost of its nature an uncertain law. For this reason, during the 20th century increasing use was made of black-letter treaties to supersede customary law. See custom.