Known to some as Eight Diagrams Theory, the bagua is one of two internationally-recognized symbols of ancient Chinese philosophy (the first is the taiji).

There are two baguas: Early Heaven or preheaven (Hetu) and Later Heaven or postheaven (Luoshu).

As used in McFengshui

The bagua invented by Lin Yun has very little in common with the original versions and should not be confused with them.

As used in authentic feng shui

The baguas form the basis of forecasts in authentic feng shui. The astronomy, numeric values and attributes, and other correspondences in use since the Neolithic are still applied in traditional feng shui.

According to the Appended Words Commentary (c. 300 BCE) the ancients used astronomy as the basis for the symbols in the Yijing, including the bagua. Both bagua are ancient star maps.

The writing that Fu Xi saw on the turtle’s back provided

a list of the precise degree coordinates for Ursa Major and the xiu, the calculations requisite for recording the ascent and decline of kingdoms.

— Cheng Zhuan, in the Lun Yu

Kun, which is north in Early Heaven and southwest in Later Heaven, is called River (chuan) in the Mawangdui version of the Yijing. The hexagram illustrates a cataclysm of water, yin in its astronomical meaning.

Tian Han, the Heavenly River (Milky Way) broke free of its banks and flooded the world. It obscured the celestial marker for the vernal equinox and severed the connection between heaven and earth, sometime in the 6th millennium BCE.

Qian represents the bridging of the gap (Tianjin, the Heavenly Ford).

Kun and Qian are wet and dry. The change that is Dao is transformation from wet to dry and back again, as Deborah Lynn Porter notes in From Deluge to Discourse.

Other applications

Liu Zihua claimed in the 1930s to have used Eight Diagrams Theory to discover a tenth planet in the solar system.

Xu Daoyi, a former researcher at the China Seismology Administration, claims his unique mix of seismological science and Eight Diagrams Theory allows him to predict earthquakes.