In this period, Chou's inspirational source came from the philosophical book, the "I Ching" ("Book of Changes"), the contents of which he said represent “the germinal elements of all that happens in the universe, including natural phenomena, human affairs, and ideas.” Based on the yin and yang concepts presented in "I Ching", Chou created variable modes – a system of interval contents and pitch contents that correspond to the trigrams and hexagrams in "I Ching". Chou applied and experimented with the principles of the "I-Ching" in harmonic, thematic, textural, and rhythmic structures.