Bagua
The Ba-Gua is a systerm of trigrams (stacks of three broken and/or solid lines) designed by King Wen almost three thousand years ago to describe the nature of changes in the universe. 

The solid lines in the trigrams symbolize yang and the broken lines symbolize yin.  Thus, a trigram with two or more broken lines is primarily yin, whereas a trigram with two or more solid lines is primarily yang.  Yin type trigrams are associated with receptive energy and stillness, while yang-type trigrams are associated with expansive energy and action.

The patterns of changes represented in the post-celestial ba-gua can tell a feng shui practitioner about the type of energy, destructive or nourishing, that "enters" a particular position of a building at a particular time.

Below is a more detailed explanation of the eight (8) trigrams, or 'Ba-Gua'.
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Qian (Chien)  - is associated with persons of authority (e.g., father, employer, King) and represents Heaven.  It's number is 6 and its element is Metal.  In the human anatomy it rules the head and lungs.  This Guas direction is Northwest. 


Kun - represents the Earth, located directly opposite Qian in the Early Heaven Ba Gua arrangement.  Its element is Earth and represents the Mother and persons of a nurturing quality.  Kun rules the the stomach and flesh in the anatomy and its direction is Southwest and its number is 2. 

Zhen (Chen) - relates to the eldest osn in the family.  Its element is Wood and rules the liver and hair in the human anatomy.  Its direction is East and its number is 3. 


Xun (Sun) - relates to the eldest daughter and its element is Wood.  In the human anatomy, this Gua rules the upper arms and thighs and its number 4 and its direction is Southeast.


Kan - relates to the middle son and represents the kidneys, ears and blood.  Its direction is North and its number is 1.



Li - is the opposite of Kan and its element is Fire.  It relates to the middle daughter and in the human anatomy, it rules the heart and the eyes.  Its number is 9 and its direction is South.


Gen
(Ken) - represents the youngest son and rules the spine and bones in the human anatomy.  It's element is Earth and its number is 8 and its direction is Northeast.


Dui (Tui) - rules the youngest daughter and its direction is West.  In the anatomy, it represents the mouth, tongue and the throat.  Its element is Metal and its number is 7.

Images from <a href="http://www.feng-shui-institute.org/trigram6.htm">The Feng Shui Institute</a>