Why Are Movie Previews Called 'Trailers'?

Movie previews were originally shown at the end of a film, trailing the main feature. This is why movie previews are called "trailers."

Trailers weren't always played before movies—and the very first trailer on record wasn't even for a film. It was actually for a 1913 play called The Pleasure Seekers. In 1913, You would pay your admission—usually just a couple of cents—and you could basically sit inside a movie house all day and watch whatever was playing, often a combination of feature-length movies, short films, and cartoons. To take advantage of the audience members sitting and waiting for the next movie to play

source: mentalfloss