Unions, labor, workers' organizations formed to improve pay, working conditions, and benefits through collective power. Modern labor unions arose out of the concentrations of workers created by the Industrial Revolution. A craft (horizontal) union organizes workers by their particular skill; an industrial (vertical) union includes all workers in an industry. Unions negotiate contracts with employers by the process of collective bargaining. A closed (or union) shop increases bargaining strength by requiring that all workers belong to a union. Some states prohibit union shops by law. A dispute with employers may be referred to arbitration, or union members may resort to strikes, slowdowns, boycotts, or, more rarely, sit-downs. In the United States, local craft unions existed from the late 1700s. The influence of the socialistic Knights of Labor (1869–1917) gave way to that of the craft unions of the American Federation of Labor (AFL; founded 1886). In the early 1900s a wave of radicalism resulted in the formation of the Industrial Workers of the World, a revolutionary union across all industries. The industrial union-based Congress of Industrial Organizations (CIO) was formed in the 1930s, during the Depression. The AFL and CIO merged in 1955. The Taft-Hartley Act (1947) placed restrictions on unions, and the Landrum-Griffin Act (1959) curbed union corruption. In recent years, union organizing has lagged as the U.S. economy has become more service-oriented. Britain has a single Trades Union Congress, but many countries have rival unions with differing political outlooks.