Why were Arnold Schoenberg’s writings important to 20th-century musical thought?

Arnold Schoenberg’s writings were profoundly important to 20th-century musical thought because they provided a rigorous theoretical foundation for the transition from traditional tonality to new methods of organization, such as atonality and serialism.12

His influence was exerted through several key avenues:

• Theoretical Pedagogy: Schoenberg was a highly influential teacher whose textbooks, including Harmonielehre (Theory of Harmony), Models for Beginners in Composition, and Fundamentals of Musical Composition, codified his evolving philosophies. He famously noted that he learned as much from his pupils—most notably Alban Berg and Anton Webern—as they did from him.23
• Structural Innovation: His writings and compositions addressed the "vast structural problems" caused by abandoning tonality, leading to his invention of the 12-tone method. This provided a systematic, unifying principle for organizing music where all 12 tones are related only to one another, replacing the traditional hierarchy of key centers.45
• Musical Criticism: Schoenberg viewed his work as an extension of musical criticism. For instance, his orchestration of Brahms’s Piano Quartet in G Minor was an act of creative criticism intended to resolve issues of audibility in the original score.6

Ultimately, his work established a "dynamic dualism" between tonality and atonality that defined much of the 20th-century musical discourse.1