A libretto is the text used in, or intended for, an extended musical work such as an opera, operetta, masque, oratorio, cantata or musical. The term libretto is also sometimes used to refer to the text of major liturgical works, such as the Mass, requiem and sacred cantata, or the story line of a ballet. The relationship of the librettist (that is, the writer of a libretto) to the composer in the creation of a musical work has varied over the centuries, as have the sources and the writing techniques employed.