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:
Do not support the union.
Do not wish to pay union dues associated with union membership.