are the expression of two complementary opposites. Yin 陰 represent the aspect of quiescence, and yang 陽 the aspect of activity. Almost everything can be divided in yang and yin, but the primary dichotomies are day and night, light and dark, hard and soft, male and female and exterior and interior. It is important to understand yin and yang while being opposites, are not two separate entities. Both can be found simultaneously in the same place. It is just like two sides of a coin: that which is yang viewed from one side is yin viewed from its opposite side.
Shudo Denmei