A pre-entry closed shop is a form of union security agreement under which the employer agrees to hire union members only, and employees must remain members of the union at all times to remain employed. This is different from a post-entry closed shop, which is an agreement requiring all employees to join the union if they are not already members. In a union shop, the union must accept as a member any person hired by the employer. By comparison, an open shop does not require union membership of potential and current employees.