A closed shop is a form of union security agreement under which the employer agrees to hire union

Trade union

A trade union, trades union or labor union is an organization of workers that have banded together to achieve common goals such as better working conditions. The trade union, through its leadership, bargains with the employer on behalf of union members and negotiates labour contracts with...

 members only, and employees must remain members of the union at all times in order to remain employed.

International Labour Organization

International Labour Organization

The International Labour Organization is a specialized agency of the United Nations that deals with labour issues pertaining to international labour standards. Its headquarters are in Geneva, Switzerland. Its secretariat — the people who are employed by it throughout the world — is known as the...

 covenants do not address the legality of closed shop provisions, leaving the question up to each individual nation. The legal status of closed shop agreements varies widely from country to country, ranging from bans on the agreement to extensive regulation of the agreement to not mentioning it at all.
The Taft-Hartley Act

Taft-Hartley Act

The Labor–Management Relations Act is a United States federal law that monitors the activities and power of labor unions. The act, still effective, was sponsored by Senator Robert Taft and Representative Fred A. Hartley, Jr. and became law by overriding U.S. President Harry S...

 outlawed the closed shop in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 in 1947.