A radio trailer is a 20-25 second advertisement for an upcoming film that airs on radio stations, featuring dialogue from the film to tell listeners the certificate, release date, genre, and other key information. The trailer provides a voiceover and audio snippets to describe the film's content and tone while promoting it to potential moviegoers. Examples of effective radio trailers include those for Pacific Rim, The Great Gatsby, The Conjuring, and Gravity, each around 25 seconds and including a certificate and audio clips to generate interest in the featured movies.