Closed shop is a form of union security in which the company can hire only current union members. This was outlawed in 1947 but still exists in some industries (such as printing). They account for fewer than 5% of union contracts.
Closed shop agreements stipulating that employees had to be union members at eth time they were hired.
Closed shop is a company that hires only workers who are members of the union: illegal under the Taft-Hartley Act.