Strauss developed the theory of binary opposites, which argues that words derive meaning from their relationship with opposing words. Binary opposites are frequently used in films, especially horror, with common pairs including good vs evil, sane vs insane, and human vs supernatural. While opposites were once simple, like good vs bad in fighting films, modern films use more complex and unusual pairs, as seen in the gradual revelation of vampires vs werewolves in the Twilight series. Harry Potter also relies on the clear binary of Harry as the good guy opposed to Voldemort as the evil one.