Atonal means that a piece of music does not have a clear key or tonal centre. This can be achieved by using notes from different scales or modes, or by avoiding traditional harmonic structures. Atonal music can be disorienting and challenging to listen to, but it can also be very expressive and evocative.
Some famous examples of atonal music include:
- Arnold Schoenberg's _Pierrot Lunaire_
- Alban Berg's _Wozzeck_
- Anton Webern's _Passacaglia_
Atonal music was particularly popular in the early 20th century, but it has continued to be used by composers of all eras.