The Chicago Board of Trade (the board) (defendant) maintained and operated a grain exchange in the Chicago market. The board established a set of rules to govern transactions at the grain exchange, with different rules for each of the various types of grain sales. One sale type was for to-arrive grain, or grain that was scheduled to arrive at the exchange but was not yet available. In 1906, the board enacted a rule that prohibited participants in the grain exchange from buying or selling to-arrive grain at a price other than the price listed at the time the exchange closed for the day until the exchange reopened the next business day. The effect was to prevent after-market pricing on grain that was en route and ready to ship upon arrival but that had not yet reached Chicago. The Department of Justice (DOJ) (plaintiff) believed that the rule violated § 1 of the Sherman Act and brought a suit in district court, seeking an injunction to prevent the board’s rule from being enforced. The board argued that the rule was not anticompetitive and had, in fact, been implemented to break up a monopoly maintained on to-arrive grain by a small number of grain traders who had taken over the after-hours market. The district court granted the injunction, holding that the rule was a restraint of trade under antitrust law. The board appealed the decision.