Toward the end of the 19th century, Romanticism reached its limits of expression, which is evident in Wagner's operas. As a result, diverse and experimental music forms began to emerge, breaking away from the mainstream of Romanticism. These forms included the impressionism of Debussy and Ravel and the surrealism of Satie. The emphasis on irregular rhythms within Stravinsky's The Rite of Spring caused its first audience, in 1913, to riot. Then followed the experimentation in scales and rhythms of Bartók. But possibly the most significant in terms of lasting influence was the atonal and serial approach of Schoenberg and his followers, Berg and Webern.
A key form of music to emerge at the beginning of the 20th century was atonalism (not having any definite key). Schoenberg defined atonalism as the twelve-tone system and developed it into serial music. The twelve-tone system treated all twelve notes of the chromatic scale with equal importance, and no note could be repeated until the series had run its course. The whole series could be moved up or down, inverted or run backward. Serialism went a step further and formalized the use of rhythm and harmony as well as pitch.
