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), or struck, scraped or rubbed by hand, or struck against another similar instrument. The percussion family is believed to include the oldest musical instruments, following the human voice.
The percussion section of an orchestra, however, traditionally contains in addition many instruments that are not, strictly speaking, percussion, such as whistles and sirens. On the other hand, keyboard instruments such as the celesta are not normally part of the percussion section, but keyboard percussion instruments (which do not have 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 without an identifiable pitch.
Hornbostel–Sachs has no high-level section for percussion. Most percussion instruments (as the term is normally understood) are classified as idiophones and membranophones. However the term percussion is instead used at the lower levels of the Hornbostel–Sachs hierarchy, including to identify instruments struck either with a non-sonorous object (hand, stick, striker) or against a non-sonorous object (human body, the ground) as opposed to concussion which refers to instruments in which two or more complementary sonorous parts are struck against each other, and for other purposes. For example:
111.1 Concussion idiophones or clappers, played in pairs and beaten against each other, such as zills and clapsticks.