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 as a whole are now shown before the film starts, but a teaser trailer is normally about 30 seconds to a minute long which includes the best shots of the film into that. They are sometimes shown when the film is still in production and in the editing process as a result they sometimes feature a particular scenes and alernatae versions of scences that arent sometimes in the finished product.
An example of an early teaser trailer is superman a film by Richard Donner it was nearly a year late, it was designed to re-invigorate interest in the release.
