E. I. du Pont de Nemours and Company (du Pont) (defendant) was a manufacturer of cellophane, a clear-film material used as a wrapping for foodstuffs and other items. du Pont owned a patent for a moisture-proof version of cellophane that was especially desirable as a wrapping material. du Pont’s cellophane was very popular, and the United States (plaintiff) brought an action against du Pont, claiming that du Pont had monopolized the market for cellophane. du Pont argued that the government’s alleged product market was too narrow and that cellophane was a product within the larger market for flexible packaging materials (e.g., aluminum foil, waxed paper, and Saran wrap). During the relevant period, du Pont maintained a share of 75 percent of cellophane production in the United States but less than 20 percent of flexible-packaging production. The district court accepted du Pont’s market definition after determining that consumers had treated other flexible packaging materials as functional substitutes for cellophane. Based on this definition, the district court determined that du Pont did not possess a monopoly in violation of § 2 of the Sherman Act. The government appealed.