Interpreting the Customary Rules on Interpretation

28 Pages Posted: 19 Mar 2016 Last revised: 18 Jul 2017

See all articles by Panos Merkouris

Panos Merkouris

University of Groningen - Faculty of Law

Date Written: February 1, 2016


Lord McNair, famously stated that the ‘there is no part of the law of treaties which the text-writer approaches with more trepidation than the question of interpretation’. However, it seems that for a long time now, and especially post-VCLT (Vienna Convention on the Law of Treaties), international courts and tribunals have taken in their stride such interpretative operations without batting an eye. What is striking is that the rules concerned are the rules of interpretation themselves, and in fact not the VCLT rules, but their customary law counterparts. This is surprising for two main reasons:

1) What is being interpreted is rules of interpretation, which runs the danger of sounding and being tautological in nature, and

2) what is being interpreted is customary law, which in and of itself is quite surprising as literature seems to focus entirely on interpretation of treaties.

The present paper, aims to fill this lacuna and prove that not only interpretation of customary rules of interpretation is not problematic (it is neither tautological nor impossible), but also that it is a process that should be completely distinguished from that of formation/identification of customary international law. Whereas the latter determines the existence of a customary rule and therefore by necessity it has to grapple with ‘practice’ and ‘opinio juris’, interpretation of customary rules concerns itself with the rules after they have been formed/identified. Consequently, interpretation of customary rules is not bound by the straightjacket of ‘practice’ and ‘opinio juris’, and those rules can be allowed to breathe freely and evolve in the same way that treaty rules do. Having done that, I will then demonstrate that customary rules of interpretation have consistently been interpreted in international jurisprudence and the interpretative process is based mutatis mutandis on the same rules that are used for interpretation of treaty rules.

Keywords: interpretation, interpretation of treaties, interpretation of customary international law, customary international law, induction, deduction, assertion, Vienna Convention on the Law of Treaties, VCLT

JEL Classification: K33

Suggested Citation

Merkouris, Panos, Interpreting the Customary Rules on Interpretation (February 1, 2016). University of Groningen Faculty of Law Research Paper 2016-12, Available at SSRN: or

Panos Merkouris (Contact Author)

University of Groningen - Faculty of Law ( email )

Oude Kijk in't Jatstraat 26
9700 AS Groningen, 9712 EK
+31 (0) 50 363 8258 (Phone)

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