Taiji is born of Wu Chi. It is the origin of dynamic and static states and the mother of yin and yang. If they move, they separate. If the remain static, they combine.
Yin Yang wikipedia.org
The Dao of Taijiquan Way to Rejuvenation by Tsung Hwa Jou. Chapter Two
Cheng Tzu’s Thirteen Treatises on T’ai Chi Ch’uan (nov 1985) Cheng Man-Ch’ing, Martin Inn p. 21-23
Taiji Diagram and Yang Style by Mei Ying Sheng neigong.net