The scholarship on sources of international law has not satisfactorily answered these questions: are sources regulated by any ‘rules’? Are ‘rules on sources’ part of international law? What is the source of ‘rules on sources’? The standard answer to these questions is that sources are regulated by state practice reflected in Article 38(1) of the Statute of the International Court of Justice. This position, albeit professing to be practice-based, does not account for the existence of ‘rules on sources’, nor for their legality and source-based character. Other practice-based positions have also proven insufficient: some, postulating the legality of rules on sources, fail to address their source-based nature; others, affirming both the legality and source-based character of rules on sources, and invoking custom as source thereof, fail to substantiate their claims, which are merely asserted or based on decisions of early arbitral tribunals, thus assuming custom to be judge-made (a ‘custom in foro’). This chapter examines the suitability of custom as source of rules on sources and proposes a shift away from custom in foro towards customary international law proper, established by state practice invoking ICJ Statute Article 38(1) outside ICJ proceedings, major instances of which it surveys.