By Barry Reingold and David Chiappetta ( September 4, 2018, 1:01 PM EDT) -- In June, the U.S. Supreme Court by a 5-4 vote held that the anti-steering rules in American Express Co.'s merchant contracts — which barred merchants from encouraging AmEx cardholders to use competing credit cards to pay for purchases — did not violate antitrust laws. The decision rested on the court's analysis of AmEx's role as the provider of a credit card "transaction platform" between cardholders and merchants and its holding that the services provided by AmEx to both sides of the platform comprise a single antitrust market. This article will discuss the majority's "transaction market" analysis and its possible application to other types of two-sided markets....