Some of the most visible changes in the international legal system occur when treaties specify new rules or norms of behaviour for its members. Each of these is a component of what has been referred to elsewhere as the “normative system”, or the part of the international legal system that is quasi-legislative in character by mandating particular values and directing specific changes in state and other actors' behaviour. Yet the normative system is only part of the international legal system; we adopt the term “operating system” to refer to the other segment. This article identifies six different ways that extant provisions or changes in the international legal operating system condition international rule creation: (1) setting the parameters of acceptability, (2) clarifying credible commitment, (3) providing flexibility, (4) actor specification, (5) forum specification and (6) direct law-making.