Percussion instruments can be either pitched or unpitched. Percussion instruments with definite pitch, such as timpani or mallet instruments, are handled using normal musical notation.
When the instrument has no definite pitch, like a bass drum or snare drum, notation gets stretched. The use of vertical space to represent pitch instead is used to represent different instruments. Yet sometimes we have more percussion instruments handled by one player than can fit into the lines on a staff. In other cases, we may have only one unpitched instrument to notate, where the vertical space on a 5-line staff would be wasted.
MusicXML allows percussion music to be represented both as heard and as notated by use of several features: