Atonality and post-tonality

In the early twentieth century many composers felt that tonality — with its tonic that attracts and its dominant that pushes — had exhausted itself as the only system. They looked for ways of organising notes that did not depend on a centre, and from that comes post-tonal music: structure and colour without the tension that leads toward a tonic.

A gentle route is pandiatonicism: you freely use the notes of a diatonic scale, but without any of them acting as a tonic or resolving anything; the seven notes float together with no hierarchy. A step further is atonality, which deliberately avoids any key: no note stands out as a centre and the formulas that would create a sense of tonality are sidestepped.

The most systematic route is dodecaphony or twelve-tone serialism, formalised by Arnold Schoenberg. You build a "row": a specific ordering of the twelve chromatic notes in which none repeats until they have all sounded. That way no pitch dominates the others and the idea of a tonic disappears; the row becomes the material that structures the whole piece.

There are also symmetric scales, which divide the octave into equal steps and therefore have no clear centre. The whole-tone scale has six notes, all a whole tone apart (no semitone): because every step is identical, the ear cannot find the tonic and the sound stays floating and weightless. The octatonic alternates tone and semitone with the same effect of symmetry.

An analogy: tonal music is a story with a home you return to (the tonic), with tension and resolution; post-tonal music is more like an abstract painting, organised by other rules — symmetry, the row — rather than a single centre. There is no "home" to return to: the order comes from a different logic.