is used only to supply musical accompaniment and sound effects.

In the case of feature films made in the United States, nearly all such hybrid films date to the 1927-1929 period of transition from "silents" to full-fledged "talkies" with audible dialog throughout. It took about a year and a half for a transition period for American movie houses to move from almost all silent to almost all equipped for sound. In the interim, studios reacted by improvising four solutions: