In the 20th century, the era of "blue movies" began with the silent films of the 1920s and continued throughout the post-war era as film technology improved and equipment costs were reduced to a consumer affordable level. Particularly with the introduction of the 8mm and super-8 film gauges, popular for the home movie market. Until the advent of electronic and digital video technology, the mass production of pornographic films was tied directly to the mainstream film industry. Beginning in 1969 with "Blue Movie" by Andy Warhol, the subsequent Golden Age of Porn and more permissive legislation, a rise of adult theaters in the United States, and many other countries, developed. There was also a proliferation of coin-operated "movie booths" in sex shops that displayed pornographic "loops" (so called because they projected a movie from film arranged in a continuous loop).