Inconsistency in legal interpretation is among the most salient problems in investor-State dispute settlement (ISDS) and it is one of the key issues being addressed by the reform efforts in the United Nations Commission on International Trade Law (UNCITRAL) Working Group III. While some degree of interpretive inconsistency is endemic to any legal order, systemic inconsistency tends to undermine the basic purposes of the investment treaty regime – namely protecting and promoting foreign direct investment through predictable international legal rules and institutions. This article seeks to parse the problem of inconsistency at a more granular level, in order to distinguish between types of norms where a degree of inconsistency is (relatively) manageable and (potentially) tolerable, and those where inconsistency is unacceptable. We argue that it is with regard to structural ‘rules of the game’ where inconsistency is most destructive.