The theory of "duality" explains the inter-action of opposing types of energy or contrasting forces in nature. For example, day and night, heaven and earth, man and woman are all opposites in nature. The Yin represents feminine energy. The Yang represents the masculine energy.

  If you look at the diagram of Taiji, the mostly dark half (Yin) represents the female, yielding, passive, negative, earth, moon, dark, acquiescence, absorption, tiger, and even numbers. The mostly light half (Yang) represents male, dynamic, assertive and positive, heaven, sun, light, vigor, penetration, dragon and odd numbers.

  The two energies are opposite and yet the existence of each in dependant on the other. Upon closer evaluation of the diagram of Taiji, you will notice there is a little of each in the other. This indicates that Yin may become Yang and vice versa. The Day becomes night, the cold becomes hot, and so on. The behavior of Yin and Yang describes the structural make up of all things in nature. This ever-changing relationship describes the cycles of creation and destruction according to the Philosophy of "The Dao (The Way)." The constant alternation of everything is the structure of the universe. The concept of Yin and Yang is conveyed by the use of the "tiger and dragon" in Gongfu and by the Taiji symbol.