Yin and Yang - the Dual Principle in Nature that represents the inter-action of opposing types of energy or contrasting forces in nature - refer to heaven and earth, or yin (feminine, negative) and yang (masculine, positive).
The dark half, Yin represents the female, yielding, passive, negative, earth, moon, dark, quiescence, absorption, tiger, orange color, duad, valleys and even numbers.
The light portion Yang is male, dynamic, assertive and positive, heaven, sun, light, vigor, penetration, dragon, monad, azure color, mountains and odd numbers.
The two energies are opposite and yet mutually dependent. Yin may become yang and vice versa, just as day becomes night, cold becomes hot, and the reverse. The behavior of yin and yang describes the structure of any event or thing. Their dynamic relationship describes the operation of the Dao in its cycles of creation, and their alternating movement underlies the structure of everything in the universe. The concept of yin and yang is conveyed by the "tiger and dragon" and by the Taiji symbol.
The 'ultimate' symbol is a picture composed of the Eight Diagrams combined with the black and white 'double fish' Yin and Yang symbol in the center. According to legend, the picture was created by Fuxi, an ancient Chinese sage. The Zhou Dynasty Book of Changes (Zhou Yi) explains that all "changes originate in the Ultimate; from the Ultimate issues the two spheres of the eight trigrams. From the two spheres issues the four elements, and from the four elements issues the eight diagrams". By the Ultimate, the ancient meant the origin of all things and creatures.