Indefinite pitch refers to any sound that listeners find difficult or impossible to identify regarding its pitch, although it is sometimes possible for them to determine its approximate pitch, or at least which of two indefinite pitches is higher or lower. Thus, instruments with indefinite pitch cannot be used to play melodies, but, instead, are used for rhythm-making and special effects. They are not notated on scores using conventional notes at various heights on a five-line staff, but rather are shown on a single staff line which indicates only their rhythm and dynamics. Most percussion instruments have indefinite pitches, including castanets, cymbals, snare drums, tambourines, triangles and wood blocks.