A membranophone is a musical instrument that produces sound primarily by way of a vibrating stretched membrane. It is one of the four main divisions of instruments in the original Hornbostel-Sachs scheme of musical instrument classification. The sound is produced by a membrane, also known as a "skin" or "head," stretched over an opening. Most, but not all, membranophones are generally called drums. They are classified according to material, shape, skin(s), skin fastening, playing positions, and manner of playing. The Hornbostel-Sachs scheme of musical instrument classification divides membranophones into a numeric taxonomy based on how the sound is produced. Membranophones can also be divided into small divisions based on length and breadth of sound production. The types of drums include barrel, conical, cylindrical, footed, frame, goblet, long, vessel, and waisted. In addition to drums, there is another kind of membranophone, called the singing membranophone, of which the best-known type is the kazoo.