Gypsum board is a type of laminated wallboard used for interior walls and ceilings during building construction. The gypsum industry is highly concentrated, and gypsum itself is essentially identical regardless of the manufacturer. As a result, competition in the market for gypsum mainly exists in the form of differential pricing and delivery terms. The United States (plaintiff) brought a complaint against a group of gypsum manufacturers (defendants), alleging that the manufacturers were colluding to restrain competition in violation of the Sherman Act. Specifically, the government challenged the manufacturers’ practice of telephoning one another to confirm the prices of gypsum for specific customers. The district court issued a jury instruction, instructing the jurors to find that the manufacturers had intended to fix prices if the challenged practice actually affected prices. The jury found that the practice affected prices, and the manufacturers were convicted. The court of appeals overturned the conviction on the basis of an improper jury instruction. The government appealed.