Why is a movie trailer called a trailer?

Related questions

1. Where are PJ Trailers made?
2. How long should a trailer be for a short film?
3. How long are trailers before a film?
4. What is the difference between a teaser and a trailer?
5. What is the difference between promo and trailer?
6. What is the difference between a trailer and a semi trailer?
7. Does a movie runtime include previews?
8. What is a trailer for a movie?
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10. How long are trailers at Regal?

Tags

1. #previews
2. #trailing
3. #turns
4. #shown
5. #originally
6. #trailers
7. #feature
8. #movie
9. #called
10. #because