Structure

Zhouyi considers everything in the world as having a taiji, and purports the universe as a whole to also be a taiji. The taiji is also taken as the starting point of the universe. It is said in the Attaching Statements: "In the Yi there is the Great Ultimate (taiji) which produces the Two Forms. The Two Forms generate the Four Images, and these Four Images give birth to the eight trigrams." This alludes to the evolution of cosmological process, and it also suggests that things or events can be divided into different parts, but are continuous through parts and a whole.

The diagram to represent the Great Ultimate is the Taiji Diagram, which is composed of a white half circle and black half circle, with a moving line splitting them apart in between. Both half circles look like fishes, so the Taiji Diagram is also called yin-yang fishes. It assumes the universe starts with an original qi (psycho-material force), which is malleable to transform into yinqi and yangqi. Yangqi is light and clear, so it rises to be tian (the heavens), while yinqi is heavy and dirty, falling down to be earth. This is the cosmological starting process of the heavens and earth. In other words, everything is a taiji (great ultimate) in a whole, and it can be divided into yin and yang.

There are two groups of the Eight Trigrams: the Pre-heavens Eight Trigrams (xiantian bagua 先天八卦) and the Post-heavens Eight Trigrams (houtian bagua 后天八卦). The sequence of the Pre-heavens Eight Trigrams is: Qian , Dui , Li , Zhen , Xun , Kan , Gen , Kun , and each of them resonates to a fundamental thing-image in the universe: tian (the heavens), lakes, fire, thunder, wind, water, mountains and earth. The Pre-heavens Eight Trigrams represent the initial circumstances of nature. For the ancients, the Pre-heavens Eight Trigrams symbolized the original prototypes of the world. However, after the original qi was divided, its nature remained the same, and the Great Ultimate evolved into Two Forms, Four Images, and Eight Trigrams. It is an example of the continuity of the one and the many that nature persists through various modes.

Fu Xi's Pre-heavens Eight Trigrams

The Post-heavens Eight Trigrams are the rearrangement of the Pre-heavens Eight Trigrams. Sima Qian, the Han historian, wrote: "King Wen of Zhou rearranged Zhouyi when he was in prison." King Wen of Zhou was originally the leader of the Zhou State in the late Shang Dynasty. He was put into prison by King Zhou of Yin for seven years merely on account of his "frankness." King Zhou of Yin murdered Ji Kao, the elder son of King Wen, chopped up his corpse and made bread with it. Old King Wen, at the age of 82, was forced in great anguish to eat his own son's flesh. He then devoted the remainder of his life to the trigrams, and rearranged the Pre-heavens Eight Trigrams of Fu Xi to be the Post-heavens Eight Trigrams, later named after him.

The Post-heavens Eight Trigrams start from the East, and are arranged clockwise. They correspond with the directions and seasons, as well as to the movement of the Big Dipper. Therefore, the appliance of the Zhouyi is based on the Post-heavens Eight Trigrams and extends to the ancient knowledge of astrology, geography, music, military strategy, mathematics, medicine, fengshui (literally "wind and water") and even pills of immortality. The Post-heavens Eight Trigrams created by King Wen of Zhou are said to be the theoretical foundation for the ancient Chinese pragmatic culture.

Zhouyi was created in the area of the Yellow River. Ancients observed the changing phenomena in the sky and on the earth over thousands of years, and developed a theory of Five Elements (wuxing) which one produces and conquers another based on their living experience. Wood, Fire, Earth, Metal and Water correspond with the four directions (east, west, north and south) and the center. In the sequence of wood-fire-earth-metal-water, each is the cause of its successor, and each destroys its successor's successor. In other words, the sequence of generating is: wood-fire-earth-metal-water-wood; whereas the sequence of destruction is: wood-earth-water-fire-metal-wood. These patterns help to explain the Post-heavens Eight Trigrams.