Pitching of percussion instruments is achieved through a variety of means.
Membranophones (such as timpani) are tuned by altering the surface tension of the face that is struck.
Idiophones (such as crotales) gain their pitch through the physical characteristics (such as composition, density, and physical dimensions) of each respective bar.
The term pitched percussion is now preferred to the traditional term tuned percussion:
Many untuned percussion instruments (such as the bass drum) are tuned by the player, but this tuning does not relate to a particular pitch.
Untuned percussion instruments can and frequently do make sounds that could be used as pitched notes in an appropriate context.
This second consideration also means that the traditional division into tuned and untuned percussion is to some extent oversimplified:
Some percussion instruments (such as the timpani and glockenspiel) are almost always used as pitched percussion.
Some percussion instruments (particularly, many types of bell and closely related instruments) are sometimes used as pitched percussion and, at other times, as unpitched percussion.
Some percussion instruments (such as the snare drum) are almost always used as unpitched percussion.