Things come into being and exist in two modes, which are used to describe how the eight trigrams are formed. As explained in The Book of Changes: “Changes involve taiji (太极 the supreme ultimate), which produces two modes. The two modes generate the four images, and the four images give birth to the eight trigrams.” Taiji divides itself into two mutually complementary but opposite parts, or modes. Ancient Chinese had different views as to what the modes represented. Some believed that from the point of view of the formation of the universe, the two modes could be understood as heaven and earth or as yin and yang. Others thought that as a term in divination, the two modes could refer to two groups formed by randomly dividing up 49 yarrow stalks used in divination, or the two lines, solid or broken, in the hexagrams of The Book of Changes.
Once the primordial chaos divided itself, there came into being heaven and earth. Therefore it is said in The Book of Changes that taiji (the supreme ultimate) gives birth to the two modes. (Kong Yingda: Correct Meaning of The Book of Changes)
When yin and yang appeared, the two modes emerged. (Zhou Dunyi: The Taiji Diagram Explained)