A trailer or preview is an advertisement or a commercial for a feature film that will be exhibited in the future at a cinema. The term ""trailer"" comes from their having originally been shown at the end of a feature film screening. That practice did not last long, because patrons tended to leave the theater after the films ended, but the name has stuck. Trailers are now shown before the film (or the A-movie in a double feature) begins.Movie trailers have now become popular on DVDs and Blu-rays, as well as on the Internet and mobile devices. Of some 10 billion videos watched online annually, film trailers rank third, after news and user-created video. The trailer format has also been adopted as a promotional tool for television shows, video games, books, and theatrical events/concerts.