The two-step methodology for the identification of general principles of law deriving from domestic legal systems, consisting of a comparative analysis followed by a transposability test, seems accepted as the undisputed methodology in the current work of the International Law Commission on the topic. This article examines whether this two-step approach finds reflection in the practice of and before the PCIJ/ICJ and in international legal scholarship. The analysis finds that judicial practice does not entirely follow these two steps, but the method is widely upheld in doctrinal writing. The article argues that the decision to codify this two-step methodology can be viewed as progressive development by the Commission, and may signify the crystallization of this method of identification of general principles of law.