20th-century classical music
Schoenberg's music became very atonal (not in any key). Eventually he developed what he called twelve tone music. This was atonal music which was organized by putting the notes of a musical idea in a particular order which could be changed in many ways during the piece. This way of organizing music is called “serialism” (a “series” is a “row of things”). Many composers were influenced by Schoenberg, especially Alban Berg and Anton von Webern.