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The Rules of Interpretation of Customary International Law

Periodic Reporting for period 1 - TRICI-Law (The Rules of Interpretation of Customary International Law)

Reporting period: 2018-05-01 to 2019-10-31

In international law, interpretation is ubiquitous and is the process through which the interpreter attempts to determine the true meaning of the rule that is being interpreted. Most cases brought before international courts and tribunals deal one way or another with questions of interpretation. This process has been codified in Articles 31-33 of the Vienna Convention on the Law of Treaties (VCLT). Based on these articles, some of the elements that are taken into consideration are the text, and context of the treaty, its object and purpose and the intention of the parties. However, a key issue is that these VCLT articles refer only to interpretation of treaties and not of Customary International Law.

Customary international law (CIL) , in turn, is one of the formal sources of international law and together with treaties are the most important ones, creating binding rules of international law. Some of the most crucial rules of international law started and continue to exist as CIL. The issue with CIL, however, is that it is an unwritten source of international law. Its existence is determined inductively through examination of two elements, state practice and opinio juris (acceptance as law).

Whereas in the application of treaties the process of interpretation is one that always yields a solution, with respect to CIL these rules of interpretation have not been examined. This leads to one of the following two paradoxical scenarios. Either CIL needs to be induced each and every time, by reference to state practice and opinio juris (but this is extremely problematic as it fails to take into account the continued existence, development and manifestation of CIL rules); or, even worse, CIL is asserted by international judges. But assertion, essentially means that international judges create law: they become law-makers and exercise a power to legislate (pouvoir de légiférer). Evidently in the study of CIL there is a critical gap in understanding how CIL can be applied in individual cases once it has been formed. TRICI-Law thus aims to prove that even in the case of this unwritten source, i.e. customary international law (CIL), there are rules of interpretation similar to those that exist for the interpretation of treaties, and to determine the content of these rules.

TRICI-Law consists of four Work Packages:

Interpretability of CIL in Theory (WP1)
This WP will examine the theoretical validity of CIL being open to interpretation. It will do so by examining international legal theory with respect to both CIL and interpretation. It will provide answers as to whether and why doctrine and domestic legal practice allow or disallow the possibility of interpretation of CIL. In the latter scenario, alternative procedures will be identified that can account for the gap that is currently observed in international judicial practice.

Rules of Interpretation of CIL in International Judicial Practice (WP2)
This WP will examine the case-law of international judicial and quasi-judicial bodies, in order to induce from it the rules of interpretation of CIL and their precise content. It will provide answers not only as to the exact content of these rules, but also as to their nature. These rules of interpretation may be legal stricto sensu, but they may also be principles/approaches/maxims/rules of logic, which also have content but may not be binding per se on the interpreter.

Convergence/Divergence of Rules of Interpretation and their Evolution through Time (WP3)
This WP will examine points of convergence/divergence between the rules of interpretation of CIL and those of treaties and of unilateral acts. It will provide answers as to what the similarities and the differences are in both the nature and the content of these rules and inquire into the reasons that may explain these points of convergence/divergence. Furthermore, it will examine the points of convergence/divergence between these rules not only as they stand now, but as they have evolved through the passage of time.

Articles/Guidelines on Interpretation of CIL with Commentary (WP4)
WP4 will be a synthesis WP, bringing together all the findings of the previous WPs, and condense them in a single, user-friendly document. That document will be in the form of articles/guidelines on interpretation of CIL and will be accompanied by a more extensive article-by-article commentary.
Panos Merkouris (PI) has published 3 articles/chapters, 3 reports and 1 blog-post.
Nina Mileva (PhD 1) has published 2 articles/chapters.
Marina Fortuna (PhD 2) and Nina Mileva (PhD 1) have co-authored and published 1 article/chapter and 1 blog-post.

Panos Merkouris (PI) has been:
a. the speaker in 4 conferences/workshops/seminars;
b. the invited speaker in 17 conferences/workshops/seminars;
c. the chair in panels of 3 conferences/workshops/seminars;
d. visiting lecturer at the University of Athens, and at Queen Mary, University of London.
e. apart from the above conferences/workshops, he has also attended/participated/contributed to 2 conferences/workshops/seminars.

Nina Mileva (PhD 1) has been:
a. the speaker in 4 conferences/workshops/seminars;
b. the invited speaker in 1 conferences/workshops/seminars;
c. apart from the above conferences/workshops, she has also attended/participated/contributed to 4 conferences/workshops/seminars.

Marina Fortuna (PhD 2) has been:
a. the speaker in 5 conferences/workshops/seminars;
b. apart from the above conferences/workshops, she has also attended/participated/contributed to 3 conferences/workshops/seminars.


A. Conferences/Workshops/Seminars
1. TRICI-Law & GLOBTAXGOV, Seminar: International Tax Governance and the Digitalization of the Economy in a Broader Perspective (16 October 2019)
2. TRICI-Law & ESIL IGILTP, The Theory and Philosophy of Customary International Law and its Interpretation – 1st TRICI-Law & ECTPIL Conference (24-25 May 2019)
3. TRICI-Law & GLOBTAXGOV, Conference: The Belt and Road Initiative (21 February 2019)
4. TRICI-Law & GLOBTAXGOV, Workshop: Customary International Law and its Interpretation in International Tax & Investment Law (20 February 2019)

B. Other Events
1. Visit to the International Court of Justice (ICJ) (22 November 2019)
2. Visit to the International Criminal Court (ICC) & Attendance of the Ongwen Hearings (22 November 2019)
3 P Merkouris & NEXUS, Careers Day 2.0: International Courts and Tribunals (16 May 2019)

1. Creation of an institutional website for TRICI-Law: (
2. Creation of a Facebook page: (
3. Creation of a Twitter account (@TRICI_Law): (
4. Creation of a Youtube channel: (

1. P Merkouris, Open Access Ambassador (31 October 2019)
2. P Merkouris, Young Academy Groningen – Selected Member/2019 Cohort (30 September 2019)
For the progress of the project beyond the State of the art, see publications and all other activities described in detail in the Section 'Work Performed from the Beginning of the Project'.
For expected results until the end of the project, see the description of the four Work Packages in the Section 'Summary of the Context and Overall Objectives of the Project'.