What is the difference between a treaty and customary international law as sources of international law?

• A. Treaties are unwritten norms that do not require ratification; customary international law is the formal written agreement.
• B. Customary international law is binding only on colonial powers.
• C. Treaties are oral agreements that states may ignore; customary law is created by parliaments.
• D. Treaties are written agreements binding on parties that ratify them; customary international law arises from general, consistent state practice accompanied by a belief it is legally obligatory.

Answer: (D)

Treaties are written agreements negotiated and consented to by states. They bind only those states that ratify or accede to them and they come into force under the terms of the treaty.

Customary international law, by contrast, is unwritten and forms from a general and consistent pattern of state practice accompanied by a belief that such practice is legally obligatory (opinio juris). Because it rests on belief in legal obligation, it binds all states regardless of whether they joined a particular agreement.

Treaties can be bilateral or multilateral and can be limited to specific issues; customary law develops gradually and can evolve as practice and beliefs change. When there is a treaty on a subject, the treaty governs the parties to it, while customary law can apply more broadly, including to states that are not parties to the treaty and can fill gaps where no treaty exists.