Answer
In 1907, Arnold Schoenberg published his Sketch of a New Esthetic of Music. This text is one of the most important works of musical theory in the 20th century, and it laid the groundwork for Schoenberg's later development of atonality. In the Sketch, Schoenberg argues that the traditional tonal system had become exhausted, and that new musical forms and techniques were needed to express the modern world.