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Max Planck Encyclopedia of Public International Law [MPEPIL]

Customary International Law

Tullio Treves

From: Oxford Public International Law (http://opil.ouplaw.com). (c) Oxford University Press, 2023. All Rights Reserved.date: 22 May 2024

Customary international law — State practice — Opinio juris — Codification — Soft law — Judicial decisions — General principles of international law — Responsibility of international organizations — Membership of international organizations — Treaties, effect for third states — Vienna Convention on the Law of Treaties — State succession, international agreements

Published under the auspices of the Max Planck Institute for Comparative Public Law and International Law under the direction of Professor Anne Peters (2021–) and Professor Rüdiger Wolfrum (2004–2020). 

1 The expression ‘customary international law’ concerns, on the one hand, the process through which certain rules of international law are formed, and, on the other, the rules formed through such a process. While these rules are not necessarily general in scope, all existing general rules of international law are customary (see paras 35–40 below; see also General International Law [Principles, Rules and Standards]).2 Even though language is necessary to communicate their content, expression through language is not an indispensable element of customary...
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