where line workers
are represented by a union
or unions, involving collective bargaining
and the like.
In states without right-to-work laws, a union shop will often be a closed shop, where workers are required to join the union representing their particular job specialization. In states with right-to-work laws, workers cannot be forced to join a union, and therefore all workplaces are technically open shops. (Of course, not joining the union in an open shop may alter your working conditions significantly, including pay and relations with fellow workers.)