The “Sources for Treaty Interpretation” and How to Use Them, According to the Vienna Convention on the Law of Treaties

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Treaties require interpretation, and there are many controversial questions about what we might call the “sources for treaty interpretation”. What materials can be used to interpret a treaty? And how are they to be used? (These questions are quite distinct from the traditional questions about the “sources of international law”.) For answers to these questions, many international tribunals now rely on the Vienna Convention on the Law of Treaties [VCLT]. This paper discusses a fundamental question about the structure of the VCLT, a question that, remarkably, has received virtually no explicit attention. What is the rationale of the distinction between Article 31 (“General rule of interpretation”) and Article 32 (“Supplementary means of interpretation”)? Why are there are two separate Articles here, and how do they differ? As we shall see, the two Articles contemplate very different uses for the materials they identify, and that is the key to deciding which materials come under which Article. Understanding the rationale for the distinction between the Articles is essential to understanding particular provisions of the Articles, such as 31.3(c). It is thus central for treaty interpretation.


A shorter verion of this paper is forthcoming as a chapter in Samantha Besson and Jean d'Aspremont, eds., The Oxford Handbook on the Sources of International Law (forthcoming from OUP 2017).

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