The Taiji (Great Heavenly Axis) forms a unity, from which two antagonistic concepts, Yin and Yang originate. The word Yin originally referred to a hillside facing away from the sun. Philosophically, it stands the gloomy, passive, female concept, whereas Yang (the hillside facing the sun) stands for the bright, active, male concept. Both concepts, though antagonistic, are also complementary and the present domination of one implies the future rise of the other, as moon's phases (this is one of the meanings of the well-known Yin-Yang figures).

This is the Yin-Yang symbol or Taijitu picture, with black representing Yin and white representing Yang. It is a symbol that reflects the inescapably intertwined duality of all things in nature, a common theme in Taoism. No quality is independent of its opposite, nor so pure that it does not contain its opposite in a diminished form: these concepts are depicted by the vague division between black and white, the flowing boundary between the two, and the smaller circles within the large regions.