Columbia Broadcasting Systems employed television technicians and stage workers who fought over the lighting work for television shows. The Radio and Television Broadcast Engineers Union, Local 212 (defendant), represented the technicians, but stage workers belonged to another union. Both unions had bargaining-agent certifications for their respective workers and collective-bargaining agreements (CBAs) with Columbia, but none of the contracts clearly apportioned the lighting work. For years the parties quarreled over remote lighting for telecasts broadcast away from Columbia’s home studio. Each union called strikes when Columbia assigned lighting work to the other’s members. When Columbia assigned remote-lighting work for a major telecast from New York’s Waldorf-Astoria Hotel to stage employees, television technicians refused to operate the cameras, forcing Columbia to cancel the show. Columbia filed unfair-labor-practice charges against the technicians’ union. The National Labor Relations Board (NLRB) held a hearing and found the technicians’ union did not have rights to the work under any outstanding NLRB order, certification, or CBA, but did not consider other criteria like Columbia’s past practices or industry custom and failed to affirmatively allocate the work. The technicians’ union refused to comply, arguing the NLRB had to make a final, binding determination using the same factors as in arbitration proceedings. The NLRB ordered the technicians’ union to stop striking, but the Second Circuit agreed that the NLRB had failed to make the required determination and refused to enforce the order. The Supreme Court granted review.