Connecticut general contractor Frouge Corporation contracted to build a Philadelphia housing project. Frouge’s collective-bargaining agreement (CBA) with the Carpenters’ International Union required Frouge to follow local union rules and regulations. In Philadelphia, the local union agreement prohibited members from handling “any doors which have been fitted prior to being furnished on the job.” Instead, Philadelphia carpenters traditionally cut and fitted blank doors on the jobsite. The union even refused to hang prefabricated doors with a union label or made by union members elsewhere. Frouge ordered prefitted doors from a manufacturer that belonged to the National Woodwork Manufacturer’s Association (NWMA) (plaintiff), and the carpenters refused to hang them. Frouge withdrew the prefabricated doors and substituted blank doors cut and fit on the jobsite. The NWMA filed unfair-labor-practice charges claiming the no-prefitted-doors rule and making Frouge reject them violated the National Labor Relations Act (NLRA). The National Labor Relations Board (NLRB) (defendant) dismissed the charges, reasoning that the provision served to protect and preserve work union carpenters did on the jobsite. The Seventh Circuit reversed, and the Supreme Court granted review.