With no reason to think that the absent class members ever agreed to class arbitration, it is far from clear that they will be bound by the arbitrator’s ultimate resolution of this dispute. Arbitration “is a matter of consent, not coercion,” Volt Information Sciences, Inc. v. Board of Trustees of Leland Stanford Junior Univ., 489 U. S. 468, 479 (1989) , and the absent members of the plaintiff class have not submitted themselves to this arbitrator’s authority in any way.

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The distribution of opt-out notices does not cure this fundamental flaw in the class arbitration proceeding in this case. “[A]rbitration is simply a matter of contract between the parties,” First Options of Chicago, Inc. v. Kaplan, 514 U. S. 938, 943 (1995) , and an offeree’s silence does not normally modify the terms of a contract, 1 Restatement (Second) of Contracts §69(1) (1979). Accordingly, at least where absent class members have not been required to opt in, it is difficult to see how an arbitrator’s decision to conduct class proceedings could bind absent class members who have not authorized the arbitrator to decide on a classwide basis which arbitration procedures are to be used.