A part may indicate the line or music intended for a particular performer. Earlier choral music, for example, was written in separate part-books, one for each part, as is the modern practice with orchestral parts, rather than in the full vocal score now usual. The art of part-writing or, in American, voice-leading, is the art of writing simultaneous parts according to the established rules of harmony. A part-song is a vocal work in which different voices are used, as distinct from a song in which all sing the same melody.