Glass's music for his ensemble culminated in the four-hour-long "Music in Twelve Parts" (1971–1974), which began as a single piece with twelve instrumental parts but developed into a cycle that summed up Glass's musical achievement since 1967, and even transcended it—the last part features a twelve-tone theme, sung by the soprano voice of the ensemble. "I had broken the rules of modernism and so I thought it was time to break some of my own rules", according to Glass. Though he finds the term minimalist inaccurate to describe his later work, Glass does accept this term for pieces up to and including "Music in 12 Parts", excepting this last part which "was the end of minimalism" for Glass. As he pointed out: "I had worked for eight or nine years inventing a system, and now I'd written through it and come out the other end." He now prefers to describe himself as a composer of "music with repetitive structures".