A trailer is a wheeled vehicle that can’t move on its own—it requires being pulled by a truck, car, or other vehicle. But when it comes to movie previews, the term has another meaning: the trailer is a film’s pre-release ad that is shown in theaters before the feature movie it promotes. Trailers are often highly polished, and they are able to present even mediocre films in attractive lights.

The trailer is a great example of the kind of digressive paratextual practice that Vincenz Hediger describes as “performative interpretation.” A good trailer is not simply a marketing tool to promote a film, but it is an interpretive space that plays with the viewer’s expectations. It manipulates the viewer’s sense of genre, narrative, spectacle, and space and time – and in doing so it makes us want to see the actual film.

It’s not uncommon for trailers to include scenes that aren’t in the final film, to shift the order of shots or edit them to create a more dramatic impact, and to use music not used in the actual film to elicit a specific reaction from viewers. Sometimes the lies are so flagrant they actually give away plot details (see: Rogue One), or a key character is omitted altogether (the most recent example being Nicolas Winding Refn’s Drive, which left out Spider-Man from its trailers).

For the average cinemagoer, the trailer is an integral part of the viewing experience. It is a chance to build anticipation for a movie, and it is this sense of shared thrill that can bring people together in the theater. A good trailer is a wonderful Schrodinger’s Cat: it can be dead or alive, and you won’t know until you watch the movie.

In the early days of cinema, a full-length film would be followed by two or more shorter teaser trailers. Now, a film will typically be preceded by a short trailer that is anywhere from 45 seconds to two and a half minutes long. Each trailer version is also shortened for different platforms: Twitter and Facebook use one minute, YouTube uses two minutes, and Instagram has only 15 seconds (the same limit as its Stories).

The trailer’s importance is highlighted by the fact that it is the only form of advertising for a film that can be legally displayed in the United States without breaking the Fair Use doctrine. As such, it is a valuable piece of film art that should be studied for its ability to craft a specific mood, and for how it evokes an audience’s desire to go to the movies.