 It is simply a convention that indicates that the lines and spaces of the stave are each assigned to a percussion instrument with no precise pitch.With the exception of some common drum-kit and marching percussion layouts, the keying of lines and spaces to instruments is not standardised, so a legend or indications above the stave are necessary to indicate what is to be played.Percussion instruments with identifiable pitches do not use the neutral clef, and timpani (notated in bass clef) and mallet percussion (noted in treble clef or on a grand stave) are usually notated on different staves than unpitched percussion.