A percussion instrument is a musical instrument that is sounded by being struck or scraped by a beater including attached or enclosed beaters or rattles struck, scraped or rubbed by hand or struck against another similar instrument. Excluding zoomusicological instruments and the human voice, the percussion family is believed to include the oldest musical instruments. In spite of being a very common term to designate instruments, and to relate them to their players, the percussionists, percussion is not a systematic classificatory category of instruments, as described by the scientific field of organology. It is shown below that percussion instruments may belong to the organological classes of ideophone, membranophone, aerophone and cordophone. The percussion section of an orchestra most commonly contains instruments such as the timpani, snare drum, bass drum, tambourine, belonging to the membranophones, and cymbals and triangle, which are idiophones. However, the section can also contain aerophones, such as whistles and sirens, or a blown conch shell. Percussive techniques can even be applied to the human body itself, as in body percussion. On the other hand, keyboard instruments, such as the celesta, are not normally part of the percussion section, but keyboard percussion instruments such as the glockenspiel and xylophone (which do not have piano keyboards) are included. Percussion instruments are most commonly divided into two classes: pitched percussion instruments, which produce notes with an identifiable pitch, and unpitched percussion instruments, which produce notes or sounds in an indefinite pitch. Function Percussion instruments may play not only rhythm, but also melody and harmony. Percussion is commonly referred to as "the backbone" or "the heartbeat" of a musical ensemble, often working in close collaboration with bass instruments, when present. In jazz and other popular music ensembles, the pianist, bassist, drummer and sometimes the guitarist are referred to as the rhythm section. Most classical pieces written for full orchestra since the time of Haydn and Mozart are orchestrated to place emphasis on the strings, woodwinds, and brass. However, often at least one pair of timpani is included, though they rarely play continuously. Rather, they serve to provide additional accents when needed. In the 18th and 19th centuries, other percussion instruments (like the triangle or cymbals) have been used, again generally sparingly. The use of percussion instruments became more frequent in the 20th century classical music. In almost every style of music, percussion plays a pivotal role. In military marching bands and