Instruments that are sounded by striking, shaking, plucking, or scraping. All instruments such as drums and bells fall into this category. The formal classifications of most Percussion instruments are either Idiophones (instruments that vibrate when struck, shook, plucked, or scraped) or Membranophones (instruments that have a stretched membrane that vibrates when struck, shook, or rubbed). Informally, Percussion instruments may be further divided into those instruments that produce a definite pitch and those that do not. Some whistles (aerophones) are also included in this category of instruments because they tend to be considered sound effects rather than serious instruments.