Why are coming attractions called "trailers"? If they come before, how can they be trailing?
-- Meetoo, the net
Once upon a time, looooong ago in Hollywoodland, movie theaters showed two (yes, two!) feature films. Really. It's true. Two! Plus newsreels! Cartoons! Weekly adventure serials! When you gave up 25 cents for a ticket, you were set for the whole afternoon. In the projection booth, coming attractions were hitched to the end of the first feature and acquired the name trailers because of their physical location on the reel. Today they have to show trailers up front with the rest of the advertising junk while we're still a captive audience.