This article proposes a new model for the engagement of sub-state units with the international legal order. “Trialogical subsidiarity” acknowledges that some areas are best regulated locally, but it also argues that international law has an increasing say in areas traditionally reserved for local law. The implementation of an international cultural heritage treaty by constituent units (CUs) in federal states, despite objections of the federal authorities, is a case study for the possibilities and implications of the use of international law by CUs without the filtering of the central state. This use enhances the legitimacy of international law and can lead to better outcomes for local populations, moving international law closer to its promise of being a law of peoples rather than of states.
