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The piece of music which plays over the opening and/or closing credits of a TV show, and is intended to become intimately identified with the series. Theme tunes began in the earliest days of radio, where there were no specific channels to switch to and listeners would often have to tune their crystal sets with some precision in order to pick up the correct station, which may have been located hundreds of miles away. A theme tune allowed them to select the correct station. Shortwave radio stations still use theme tunes, called interval signals, at the beginning of their transmissions. Well-known interval signals include the Voice of America's Yankee Doodle and the BBC World Service's Lilliburlero. Theme songs are usually original works, but some shows use a song that has already been recorded (see Real Song Theme Tune). A theme song may be an instrumental or have lyrics, although most dramatic shows (including, as far as America is concerned, those animated) use an Instrumental Theme Tune. Sitcom theme song lyrics have gone through various phases. Radio theme songs were generally instrumental, possibly because it was hard to hear lyrics over music on old low-fidelity radio sets. Sitcoms that moved to television kept their old instrumental tunes, while new sitcoms created for television could choose an instrumental tune or an Expository Theme Tune. From the mid-1970s to the mid-1990s most sitcoms featured a Thematic Theme Tune. The current trend is toward the Surreal Theme Tune or No Theme Tune whatsoever. It's also possible for theme tunes to be replaced. See Leitmotif for character themes or music for recurring events. For other themes see Central Theme.
Specific types of Theme Tune include: