The study of the 8 Trigrams is an essential fundamental in Feng Shui knowledge
A Trigram, or 'Gua', consists of a set of triple lines, where each line is either solid or broken. The solid lines denote the polarity of Yang (active, positive, bright) while the broken lines are Yin (reactive, negative, dark). Yin and Yang complement each other, and by no means are one better than the other.
The possible combinations of these Yin and Yang lines in sets of 3, result in the formation of the eight (8) trigrams, or 'Ba Gua'. These are Qian, Kun, Zhen, Xun, Kan, Li, Gen and Dui.
These eight trigrams are further arranged into the Early Heaven and Later Heaven sequences:
The Early Heaven Ba Gua arrangement illustrates a state of perfection. All phenomena are in total balance, and therefore static - there is no birth, growth, or death. This arrangement is also known as the Fu Xi Ba Gua, named after the sage who founded this concept.
The Later Heaven Ba Gua, or King Wen's Ba Gua, dictates the cyclic condition of change and growth. Everything is this universe is constantly changing and goes through the motions of Birth, Growth, Decay, and Death. Within the cyclical nature of phenomena, Time and direction also exists.
Each Gua in itself represents a specific kind of condition or phenomena. Each Gua has its own element and direction, and can signify different people, body parts, and number (among many other things).
The Qian Gua is associated with persons of authority - King, Father, employer. Most importantly Qian represents Heaven. Its number is 6 and its element is Metal. In the human anatomy it rules the head and lungs. The direction of Qian is Northwest.
Kun Gua, which represents the Earth, sits opposite Qian in the Early Heaven Ba Gua arrangement. Heaven and Earth complement each other to form a state of perfection. Kun Gua, belonging to the element of Earth, represents the Mother and persons of a nurturing quality. Kun rules the stomach and flesh in the anatomy, and the Southwest direction. Its number is 2.
Zhen Gua, the phenomenon of Thunder, relates to the eldest son in a family. Its element is of Wood, and in the anatomy Zhen Gua is the voice, the liver, and hair. Its direction is the East, and its number is 3.
Xun Gua, which is Wind, complements Zhen in the Early Heaven arrangement. Xun also belongs to the element of Wood, but as opposed to Zhen's wood that grows strong and unbending like a tree, Xun wood is like a leaf or plant that can bend gracefully with the wind. In a family, Xun is the eldest daughter, and in the human anatomy, Xun is the upper arms and thighs. Its number is 4, and its direction is Southeast.
The trigram Kan rules over Water and its direction is North. The phenomenon of Kan, water, is also its element. In the family Kan is the middle son, and in the anatomy it represents the kidneys, the ears, and blood. Its number is 1.
Li Gua, the opposite of Kan, is Fire. It represents the middle daughter, the heart and eyes, and also the number 9. Its direction is South. In the Early Heaven format, Kan Gua and Li Gua sit in perfect harmony. One may wonder how does fire and water complement each other - the reason is simple: When there is too much of water, it becomes too cold, and things will drown or rot. Similarly, when there is too much fire, it burns everything up. Therefore, a balanced amount of fire and water will keep each other in check.
Gen Gua, representing Mountains, belong to the element of Earth. In the family it is the youngest son; in the anatomy it is the spine and bones. Its number is 8 and its direction is the Northeast. Gen Gua is complemented by Dui Gua's Lake. Dui rules the youngest daughter, and the direction of West. In the anatomy it represents the mouth, the tongue, and the throat. The element of Dui is Metal, and the number 7 signifies it.
The meanings derived from these trigrams are infinite, because indefinitely, every event in the universe can be associated with one of the trigrams. The information portrayed above, along with the impressions and symbolisms are ones commonly used in the study and application of Feng Shui. It is also interesting to note that, while Feng Shui theories are derived from the Early Heaven formation (the static nature of the universe), application is based on the Later Heaven formation (the cyclical nature of the universe).
The study of trigrams is especially important in the San Yuan (Triple Time Cycles) school of Feng Shui, as opposed to the San He (Three Combinations) school, which rely more heavily on stems and branches (akin to Four Pillars astrology).