Beginning in the late 1950s, Chou began to experiment with variable modes in his compositions. Jianjun He classified Chou's works into two categories: “pentatonic-related” or “variable modes-based” composition. Most of Chou's early works are pentatonic-related and Chou drew his inspirations from traditional Chinese pentatonic melodies. The piece "metaphor" (1960) marks the beginning of Chou's middle period when Chou utilized the variable modes as a compositional method to pitch organization. Later, Chou applied the modes to other works such as "Cursive" (1963), "Pien" (1966), and "Yun" (1969). Chou stated that the structure of "Pien" is based on the concept of balance between the positive and the negative forces as stated in I-Ching.