The arrangement described restricts employment to individuals who are already members of a specific labor union. This means that to be hired and maintain employment, workers must join and remain in good standing with the designated union. For example, a construction company might agree with a union local to only hire carpenters who are members of that particular union.
Historically, this type of labor agreement was seen as a way to strengthen unions, giving them more power in negotiations with employers and ensuring a unified workforce. Proponents argued it prevented free-riding, where non-union members benefitted from union-negotiated wages and conditions without contributing dues or participating in union activities. Its prevalence varied significantly across industries and geographic locations, often being more common in sectors with strong union presence.