By the 1930s, however, studios and exhibitors began to realize the promotional value of trailers was great enough that they shouldn’t be burying them after the main program, but instead giving them the prime advertising slot of bumping up against the top of the movie, ensuring that patrons would be back next week, and the week after that, and the week after that. And thus the current tradition was born — even if the term attached to it was an inaccurate anachronism.
Further recommended reading on the history and form of trailers: Filmmaker IQ’s “History of the Movie Trailer,” The Dissolve’s David Fear’s “Becoming Attractions: A Brief History of Film Trailers,” and Vulture’s Matt Patches on “The Age of the Movie Trailer Money Shot.”