Add to ORKGThe orthodox view of market definition in antitrust cases is that the same principles of market definition should apply at all stages of an antitrust analysis, and, in particular, that markets should be defined for virtually all purposes by reference to demand-side substitutability. Commentators have struggled to reconcile the Supreme Court’s recent decision in Ohio v. American Express Co.—in which the majority combined services to cardholders and services to merchants into a single antitrust market, despite the evident lack of substitutability between them—with that familiar view. In this short Article, we suggest that this effort at reconciliation may be unnecessary, and perhaps even undesirable. Against the orthodox view, we claim that m...