Eisen (plaintiff), an odd-lot trader on the New York Stock Exchange, filed a class action suit against DeCoppet & Doremus and Carlisle & Jacquelin (defendant) on behalf of himself and individuals who traded odd-lot stocks between May 1, 1962 and June 30, 1966. The class was comprised of approximately 2,250,000 members. Eisen alleged that the defendants, who handled 99 percent of odd-lot trading on the Exchange, were charging an excessively high differential for trades, and that they had an unlawful monopoly over the market in violation of the Sherman Act. The defendants challenged the certification of the class under Rule 23, and the district court granted their motion to decertify the class. On appeal, the court of appeals issued two separate rulings: (1) that the district court’s decision was final and therefore properly appealable (Eisen I); and (2) that the class might be properly certified under the rule (Eisen I). The court of appeals thus remanded the case to the district court to make a finding. On remand, the district court found that the class could be properly certified, and the defendants again appealed. The court of appeals reversed the district court’s determination and denied the class certification (Eisen III). The United States Supreme Court granted certiorari to clarify several issues in the case.