Atonal music is music with no key-centeredness, or diatonic scale associated with it, and no hierarchy of pitches focusing on a single, central tone; the notes of the chromatic scale function independently of one another. The music is often considered disconcerting by those who are accustomed to "traditional" harmonies. More narrowly, the term describes music that does not conform to the system of tonal hierarchies that characterized classical European music between the seventeenth and nineteenth centuries. More narrowly still, the term is sometimes used to describe music that is neither tonal nor serial, especially the pre-twelve-tone music of the Second Viennese School, principally Alban Berg, Arnold Schönberg, and Anton Webern. Composers such as Alexander Scriabin, Claude Debussy, Béla Bartók, Paul Hindemith, Sergei Prokofiev, Igor Stravinsky, Edgard Varèse, György Ligeti, Krzysztof Penderecki, Toru Takemitsu, and Wolfgang Rihm have written music that has been described, in full or in part, as atonal. In addition to the Second Viennese School, composers associated with serialism include Olivier Messiaen, Pierre Boulez, Luigi Nono, Luciano Berio, Luigi Dallapiccola, Karlheinz Stockhausen, and Milton Babbitt.