Atonality, in music, is the absence of functional harmony as a primary structural element. The reemergence of purely melodic-rhythmic forces as major determinants of musical form in the Expressionist works of Arnold Schoenberg and his school prior to World War I was a logical, perhaps inevitable consequence of the weakening of tonal centres in 19th-century post-Romantic music. By the time of Richard Wagner’s Tristan und Isolde, for example, the emphasis on expressive chromaticism had caused successive chords to relate more strongly to each other than to a common tonic firmly established by intermittent harmonic cadences. Eventually, the chromatic scale of 12 equidistant semitones superseded the diatonic scale, the inseparable partner of functional harmony, to the extent that melodic-rhythmic tensions and resolutions took the place of the harmonic cadences and modulations that had determined the structure of Western music for centuries. Continue reading from Encyclopedia Britannica
Twelve-tone music is most often associated with a compositional technique, or style, called serialism. The terms are not equivalent, however. Serialism is a broad designator referring to the ordering of things, whether they are pitches, durations, dynamics, and so on. Twelve-tone composition refers more specifically to music based on orderings of the twelve pitch classes.
This style of composition is most associated with a group of composers whose figurehead was Arnold Schoenberg and which also included the influential composers Anton Webern and Alban Berg. But twelve-tone compositional techniques and ideas associated with such techniques were very influential for many great composers, and serial and twelve-tone music is still being written today. Much of this music shares similar axioms, outlined below, but composers have used these basic ideas to cultivate entirely original approaches.
Twelve-tone music is based on series (sometimes called a row) that contains all twelve pitch classes in a particular order. There is no one series used for all twelve-tone music; most composers write a unique row for each piece. Continue reading from Viva Open Music Theory
Atonal Music: Can Uncertainty Lead to Pleasure? (US National Library of Medicine)
Tonality in Crisis? How Harmony Changed in the 20th Century (British Library)
Arnold Schoenberg and Atonal Music Composition (Lumen Learning)
Bending an Ear to Atonal Music (Dummies)
What is Atonal Music? (Hoffman Academy)
Atonality (New World Encyclopedia)