Musical revolutions before the twentieth century — the shift from Baroque to Classical style, or Classical to Romantic — generally changed how music sounded and felt while still operating within a shared underlying system: tonality, the organisation of music around a home key that pieces establish, depart from, and ultimately return to. Some early twentieth-century composers went considerably further, deliberately abandoning tonality itself as an organising principle, and in some cases building an entirely new compositional system from scratch to replace it.
Atonality removed the home key that had organised music for centuries
Atonal music deliberately avoids establishing any single pitch or key as a clear tonal centre that the music departs from and returns to, the way tonal music consistently does. Composers including Arnold Schoenberg, working in the early twentieth century, began writing music that moved freely between pitches without the traditional hierarchical relationship to a home key, producing a sound many contemporary listeners, accustomed to centuries of tonal music, found genuinely disorienting on first encounter. This wasn't simply a new style layered onto the existing tonal system the way earlier musical shifts had been; it was a rejection of the underlying organisational framework tonal music had relied on for centuries.
Twelve-tone technique replaced tonal hierarchy with a different, deliberate structure
Schoenberg went on to develop twelve-tone technique, a compositional method that organised a piece around a fixed, specific ordering of all twelve pitches in the chromatic scale, called a tone row, used as the basis for the piece's melodic and harmonic material throughout, often manipulated through techniques like inversion, retrograde, and transposition. This gave atonal composition a rigorous, deliberate organising structure of its own, replacing tonal hierarchy's home-key logic with a different, systematic logic based on the tone row instead — a genuinely new foundational system for organising pitch, not merely an absence of the old one. Twelve-tone technique and related serial composition methods went on to influence a significant strand of twentieth-century classical composition, even as many other composers of the same broad period continued working with tonality in various modified or extended forms, meaning the century's music includes both composers who abandoned the old system entirely and composers who kept working within some version of it.
What we're still unsure about
The development of atonality and twelve-tone technique, and Schoenberg's central role in formalising the latter, are well documented in music history, and the technical mechanics of twelve-tone composition are precisely defined and not in dispute. What remains genuinely, actively debated is a matter of aesthetic and historical judgement rather than technical fact: how central twelve-tone and serial composition should be considered to twentieth-century classical music's overall story, given that plenty of significant composers throughout the same period continued working with tonality in various forms — music historians and critics don't fully agree on how much weight the atonal, serial strand deserves relative to the many composers who took a different path across the same century.
This sits inside 20th-Century Classical & Modernism, one of seven topics in Music History, one of four domains in Music, one of seventeen subjects the app can quiz you on.