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" .