A workplace where an employer:

The Labor Management Relations Act (29 U.S.C. §§ 141 to 197) amended the National Labor Relations Act to make it an unfair labor practice for employers and unions to form or bargain collectively for closed shops (29 U.S.C. §§ 158(a)(3) and(b)(3)).

In the past, a union could form a closed shop by negotiating for a collective bargaining agreement clause that required union membership as a condition of hiring and continued employment in a position represented by the union. That clause is generally called a closed shop clause and is one of several types of union security clauses, historically used to ensure that employees are excluded from the bargaining unit if they either:

For more information, see Practice Notes, Employee Rights and Unfair Labor Practices Under the National Labor Relations Act and Labor Law: Beyond the NLRA and RLA and Subjects of Collective Bargaining Chart.