Standard economics provides a well-understood framework of the competitive determinants of market prices that is now widely accepted for antitrust analysis. In “two-sidedmarkets,” where firms supply products demanded by two interrelated groups of consumers, these competitive forces operate...

I discuss the antitrust suit brought by the U.S. Department of Justice against Visa and MasterCard in 1998. Banks that issue Visa cards are free to also issue MasterCard cards, and vice versa, and many banks issue the cards of both networks. However, both Visa and MasterCard had rules...