American Express Company and American Express Travel Related Services Company (together, Amex) (defendants) provide credit-card services to merchants and consumers. Amex charges merchants higher fees than other companies but offers cardholders greater rewards. Amex merchants enter agreements with antisteering provisions that prohibit discouraging customers from using Amex cards at the point of sale. The United States and several states (plaintiffs) sued Amex, claiming that its antisteering provision violated antitrust laws. The trial court found that credit-card transactions should be treated as two separate transactions for antitrust purposes: paying the merchant and extending the customer credit. Analyzing the merchant side, the court found Amex’s antisteering provisions anticompetitive because they resulted in higher merchant fees. But the Second Circuit reversed, reasoning that the credit-card market is just one market. The United States Supreme Court granted review.