Yin and Yang

Four Laws of Yin-yang

Yin-yang are Opposing
Yin and yang describe the polar effects of phenomena. In viewing any one phenomenon (or the comparison of two phenomena), yin and yang describe the opposing qualities inherent in it. For instance, winter and summer would be the yin and yang, respectively, of the year.
Yin-yang are Mutually Rooted
Yin and yang are two complementary qualities. That is to say, the yin and the yang aspect of any one phenomenon will, when put together, form the entire phenomenon. Yin-yang is a philosophy of duality. This is the reason the Chinese word has no "and" between yin and yang - the term always expresses the two making up the one. In the example above, winter plus summer makes up the whole year.
Yin-yang Mutually Transform
The maximum effect of one quality will be followed by the transition toward the opposing quality. In other words, once the maximum Yang aspect has manifest, such as the long days of summer, this will be followed by the transition toward the Yin aspect, with the shortening of the days as winter approaches.
Yin-yang Mutually Wax and Wane
The Yin and yang aspects are in dynamic equilibrium. As one aspect declines, the other increases to an equal degree. For instance, in the cycle of the year, the long days of summer gradually shorten and the nights gradually lengthen as winter approaches. Throughout the process, however, the length of each day is constant (the equilibrium) while it is only the relative length of light and darkness that changes (is dynamic).
Yin and yang are neither substances nor forces. They are the terms used in a system of dualistic qualification which can be applied universally. By further divided the Yang and Yin aspect into their respective Yin and Yang aspects, the result is the Yin of the Yang, the Yang of the Yang, the Yin of the Yin, and the Yang of the Yin. This allows an endless scale of universally defined qualities, which is foundational to classical Chinese thought, as seen in the Tao Te Ching, and science, as seen in the Yellow Emperor's Huangdi Neijing.