The Hornbostel–Sachs system 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 lower-levels of the Hornbostel–Sachs hierarchy, including to identify instruments struck with either a non sonorous object hand, stick, striker or against a non-sonorous object human body, the ground. This is opposed to "concussion", which refers to instruments with two or more complementary sonorous parts that strike against each other and other meanings. For example: