The first film trailers began as advertisements shown between films in theaters in the 1910s. In 1919, the National Screen Service gained a monopoly on creating and distributing trailers, establishing a template that became standard. Trailers evolved from being silent to incorporating sound in 1929. Experimentation increased in the 1940s-60s as the National Screen Service's control declined. By the 1990s, trailers had become big business and focused more on safely selling expensive films than taking risks or revealing too much of the plot.