serial music, the body of compositions whose fundamental syntactical reference is a particular ordering (called series or row) of the twelve pitch classes—C, C#, D, D#, E, F, F#, G, G#, A, A#, B—that constitute the equal-tempered scale. In contrast to tonal music, whose unity is perceived in the primacy of a single construct, the triad (the major or minor chord), serial music is not pitch centric, i.e., there is no home key. Instead, the presence of harmonic successions resulting from controlled juxtaposition of various row forms gives serial pieces their coherence. These forms are the prime, retrograde (pitch order reversed), inversion (interval direction reversed), and retrograde inversion, and the twelve transpositional degrees of the foregoing. Thus, the row functions as an ordering of intervals and not of absolute pitches. In practice, the row can be presented linearly or chordally. The twelve-tone system evolved in the 1920s in the works of Arnold Schoenberg, Anton von Webern, and Alban Berg as the result of efforts to establish a unifying principle for nontonal music. Classic serial pieces include Schoenberg's Piano Suite, Op. 25 (1924) and von Webern's String Quartet, Op. 28 (1938). Pierre Boulez and Milton Babbitt have led efforts toward "total serialization," the application of serial technique to rhythm, dynamics, and timbre, in addition to pitch. Important composers of serial music include Igor Stravinsky, Ernst Křenek, Egon Wellesz, and Walter Piston. For further information see separate articles on all composers mentioned in this article.