In Chinese philosophy, the concept of yin-yang, which is often referred to in the West as "yin and yang", literally meaning "shadow and light", is used to describe how polar opposites or seemingly contrary forces are interconnected and interdependent in the natural world, and how they give rise to each other in turn in relation to each other. The concept lies at the origins of many branches of classical Chinese science and philosophy, as well as being a primary guideline of traditional Chinese medicine, and a central principle of different forms of Chinese martial arts and exercise, such as t'ai chi, and qigong. Many natural dualities such as, dark and light, female and male, low and high, cold and hot, water and fire, earth and air—are thought of as manifestations of yin and yang.
Yin and yang are not opposing forces, but complementary forces, unseen (hidden, feminine) and seen (manifest, masculine), that interact to form a greater whole, as part of a dynamic system. Everything has both yin and yang aspects as light could not be understood if darkness didn't exist, and shadow cannot exist without light. Either of these aspects may manifest more strongly in a particular object depending on the criterion of the observation. The concept of yin and yang is often symbolized by various forms of the Taijiu symbol, for which it is probably best known in western cultures.
There is a perception (especially in the West) that yin and yang correspond to evil and good. However, in Taoist (pronounced Daoist) metaphysics, good/bad distinctions and other moral judgments are perceptual and not real, and yin-yang is an indivisible whole. In the ethics of Confucianism on the other hand, most notably in the philosophy of Dong Zhongshu, a moral dimension is attached to the idea of yin and yang.
In Taoist philosophy, dark and light, yin and yang, arrives in the daodejing at Chapter 42. It becomes sensible from an initial quiescence or emptiness, which is sometimes symbolized by an empty circle, and continues moving until quiescence is reached again. For instance, dropping a stone in a calm pool of water will simultaneously raise waves and lower troughs between them, and this alternation of high and low points in the water will radiate outward until the movement dissipates and the pool is calm once more. Yin and yang thus are always opposite and equal qualities. Further, whenever one quality reaches its peak, it will naturally begin to transform into the opposite quality: for example, a sunflower that reaches its full height in summer will produce seeds and die back in winter in an endless cycle. It is impossible to talk about yin or yang without some reference to the opposite, since yin and yang are bound together as parts of a mutual whole. For example, you cannot have the back of a hand without the front. A way to illustrate this idea is to postulate the notion of a race with only men or only women; this race would disappear in a single generation. Yet, men and women together create new generations that allow the race they mutually create and mutually come from to survive. The interaction of the two gives birth to things. Yin and yang transform each other: like an undertow in the ocean, every advance is complemented by a retreat, and every rise transforms into a fall. Thus, a seed will sprout from the earth and grow upwards towards the sky – an intrinsically yang movement. Then, when it reaches its full potential height, it will fall.
Many places in China, such as Luoyang, contain the word "Yang", and a few, such as Huayin, contain the word "yin". This is a very old way to assign place names.
Classically, when used in place names, "yang" refers to the "sunny side". The word taiyang, refers to the sun, and literally means "great yang". In the northern hemisphere, sunlight comes predominantly from the south, and so the south face of a mountain, or the north face of a river valley will get more direct sunlight. Therefore, "Yang" means a place is on the south slope of a mountain (or on the north bank of a river valley). For example, Luoyang is on the north bank of the Luo River Valley.
In the same way, "yin" would be the opposite, the "shadowy side". "Yin" means that a place is on the north slope of a mountain (or on the south bank of a river). For example, Huayin is on the north slope of Mount Hua.
Yang is the white side with the black dot on it, and yin is the black side with the white dot on it. The relationship between yin and yang is often described in terms of sunlight playing over a mountain and a valley. Yin is the dark area occluded by the mountain's bulk, while yang is the brightly lit portion. As the sun moves across the sky, yin and yang gradually trade places with each other, revealing what was obscured and obscuring what was revealed.
Yin is characterized as slow, soft, yielding, cold, wet, and passive; and is associated with water, earth, the moon, femininity and nighttime.
Yang, by contrast, is fast, hard, solid, focused, hot, dry, and aggressive; and is associated with fire, sky, the sun, masculinity and daytime.
The principle of yin and yang is represented in Taoism by the Taijitu, literally meaning the diagram of the supreme ultimate. The term is commonly used to mean the simple 'divided circle' form, but may refer to any of several schematic diagrams representing these principles. Similar symbols have also appeared in other cultures, such as in Celtic art and Roman shield markings.
The Taijitu and concept of the Zhou period reach into family and gender relations. Yin is female and yang is male. They fit together as two parts of a whole. The male principle was equated with the sun: active, bright, and shining; the female principle corresponds to the moon: passive, shaded, and reflective. Male toughness was balanced by female gentleness, male action and initiative by female endurance and need for completion.
In the taijitu, the circle itself represents a whole, while the black and white areas within it represent interacting parts or manifestations of the whole. The white area represents yang elements, and is generally depicted as rising on the left, while the dark (yin) area is shown descending on the right. The image is designed to give the appearance of movement. Each area also contains a large dot of a differing color at its fullest point (near the zenith and nadir of the figure) to indicate how each will transform into the other.
The Taijitu symbol is an important symbol in martial arts, particularly Taijiquan, and Jeet Kune Do. In this context, it is generally used to represent the interplay between hard and soft techniques.
Simplified, there will always be yang, where there is yin and there will always be yin where there is yang. As I wrote earlier, yin and yang are complimentary, not opposing. One cannot survive without the other.