The gap-filling function is a prominent feature of general principles of law (GPL). However, there are several questions surrounding the prominence and characterization of this function. In light of this, this article revisits the origins of the GPL and their gap-filling function to understand why it gained prominence, and what it means to international law. By tracing the debates of the Advisory Committee of Jurists on the sources of international law, this article identifies that the necessity of gap filling is tied to the prevalent idea of the completeness of the international legal system. The article further explains the origins of the gap-filling requirement and how the GPL satisfy it. By historicizing the gap-filling discourse, the article argues that the GPL have been used to strengthen the completeness of international law.