This chapter examines the contribution that domestic courts may have in the development of rules or guidelines for the interpretation of customary international law (CIL). Through an examination of national cases where courts interpret both domestic and international custom, the chapter traces methodologies of interpretation and motivations behind them. The chapter then asks two questions: how can we learn from domestic interpretive practices? and why should we learn from them? In answer to the first question, the chapter argues that domestic courts can contribute to the development of rules for CIL interpretation beyond the role assigned to them in the framework of sources, and also in informal ways. In answer to the second question, the chapter argues that looking to domestic interpretive approaches for custom may help us fill the ‘gap’ currently existing on this issue in international law, that domestic courts offer a wealth of cases from which we can draw, and that by learning from domestic interpretive practices international law can provide domestic judges with various familiar tools for their further engagement with CIL in the future.

Chapter 5 addresses the role of the foundational principles of EU law – primacy, direct effect, consistent interpretation and state liability – in ensuring the proper functioning of the internal market. The EU-specific principles are compared with their equivalents in public international law and in the EEA, the Energy Community, the ECAA and the Transport Community legal orders. The analysis helps to determine the implications of a possible absence of the supranational principles in the agreements exporting the acquis on the effectiveness of the acquis transferred to third countries by means of agreements operating under public international law.