Many people in the labor movement and adjacent to it throw around terms like closed shop, union shop, open shop, union security clause, etc., without always using these terms correctly. There are subtle differences, and a passage from The Industrial Workers of the World: Its First 100 Years provides an example of getting closed shop and union shop wrong:
In November 1940 an 11-day strike at American Stove ended with the trading of a demand for a closed shop for the settlement of an accumulation of grievances. This was what the bargaining committee wanted, for they saw that a closed shop (unless accompanied by hiring through the union) ends up in the company personnel office eventually selecting the membership for the union.
A closed shop is not one where all employees have to become union members after being hired. That is a union shop. A closed shop is one where the employer can only hire employees through the union.
Closed shops have been illegal in the United States since 1947.
Though the above passage is technically wrong on definitions (at least in today’s usage), its implication about the difference between closed shops and union shops is significant: a great shift in the balance of power between unions and employers.
Right to work goes a step further and outlaws union shops.