What is customary international law?
"It consists of rules of law derived from the consistent conduct of States acting out of the belief that the law required them to act that way." (Rosenne, Practice and Methods of International Law, p. 55)
The elements of customary international law include:
1. Widespread repetition by States of similar international acts over time (State practice).
2. Acts must occur out of sense of obligation (opinio juris).
3. Acts must be taken by a significant number of States and not be rejected by a significant number of States.
"Customary international law develops from the practice of States. To international lawyers, the practice of states' means official governmental conduct reflected in a variety of acts, including official statements at international conferences and in diplomatic exchanges, formal instructions to diplomatic agents, national court decisions, legislative measures or other actions taken by governments to deal with matters of international concern."