Tai Chi generates two phenomena, two phenomena generates four images, and four images generate Bagua.
Liangyi refers to Yin and Yang, and Sixiang refers to Taiyin, Shaoyang, Sun and Shaoyin. Bagua refers to the eight hexagrams Qian, Dui, Li, Zhen, Xun, Kan, Gen and Kun.
The four images here are slightly different from the Eastern Green Dragon, Southern Suzaku, Western White Tiger, and Northern Black Tortoise in traditional culture. It is also different from the "earth, fire, wind and water" theory.