The hexagrams of Zhouyi have six lines, from bottom to top they are Chu, Two, Three, Four, Five and Upper (Yao). Among them, the first and second are status, the third and fourth are human positions, and the fifth and highest are heavenly positions. "Yi Zhuan" says: "The way of establishing heaven is called yin and yang, the way of establishing earth is called softness and hardness, and the way of establishing people is called benevolence and righteousness." Swearing people is not three or four, in fact, it means calling people unkind and unjust. As for seven up and eight down, because seven is yang and eight is yin, yang diverges and yin shrinks, then seven up is nine and eight down is six, and nine is the extreme of yang and six is ??the extreme of yin. Therefore, it is said: seven up and eight down.