Atonality in its broadest sense is music that lacks a tonal center, or key. Atonality, in this sense, usually describes compositions written from about 1908 to the present day where a hierarchy of pitches focusing on a single, central tone is not used, and the notes of the chromatic scale function independently of one another (). More narrowly, the term atonality describes music that does not conform to the system of tonal hierarchies that characterized classical European music between the seventeenth and nineteenth centuries (). "The repertory of atonal music is characterized by the occurrence of pitches in novel combinations, as well as by the occurrence of familiar pitch combinations in unfamiliar environments" (, 1). 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 Schoenberg, and Anton Webern (). However, "[a]s a categorical label, 'atonal' generally means only that the piece is in the Western tradition and is not 'tonal (, 1), although there are longer periods, e.g., medieval, renaissance, and modern modal musics to which this definition does not apply. "[S]erialism arose partly as a means of organizing more coherently the relations used in the preserial 'free atonal' music. ... Thus many useful and crucial insights about even strictly serial music depend only on such basic atonal theory" (, 2). Late 19th- and early 20th-century composers such as Alexander Scriabin, Claude Debussy, Béla Bartók, Paul Hindemith, Sergei Prokofiev, Igor Stravinsky, and Edgard Varèse have written music that has been described, in full or in part, as atonal (; ; ; ; ; ; ; ; ; ; ; ).

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