Interpretation of Customary Rules by Reference to Treaties and General Principles of Law

In its case law the International Court of Justice has repeatedly suggested the idea that rules of customary international law (CIL) do not operate in a vacuum but, instead, are to be understood against the background of other rules of the international legal system. This observation, although somewhat unsurprising, shows that the sources of international law exist in close interconnection – something that is also visible if one looks at the rules of interpretation contained in Article 31 of the Vienna Convention on the Law of Treaties. Accordingly, ‘any relevant rules of international law applicable in the relations between the parties’ must be taken into account, together with the context, when interpreting treaty provisions. The question addressed in this chapter is whether or not the same can be said of the interpretation of customary rules. In other words, if we look at the practice of international courts and tribunals, is it possible to reach the conclusion that CIL rules, too, must be interpreted with the cognizance of any relevant rules of international law applicable between the parties?