How did movie trailers work before the time of the internet?

I was watching some trailers for old movies and suddenly this came to my mind: nowadays we either see movie trailers on the internet or in cinemas before a movie. But back when the internet didn’t exist, how did they show trailers? Today we have digital projectors to do editing, but in the past people couldn’t do such things, especially during the 1940s – 1950s, when all people had were some film reels.