The Theory of Yin and Yang
Yin and yang are one of the most fundamental concepts in TCM because it makes up such a large chunk of the foundation of diagnosis and treatment. First appearing in the Book of Changes (Yi Jing), the theory has probably been around since prior to the Warring States Period (pre 221 B.C.). The common meaning of the traditional characters for yin and yang stand for the dark and light sides of a hill, respectively (simplified characters in parentheses).
This leads us to the basic metaphor of yin and yang being the cycle of the day, with yin being night and yang being day. The two are opposite, true. However, one will eventually lead into another in an endless cycle. Furthermore, the two define themselves by being the opposite of the other.
The ancients then looked in nature for other representations of this dichotomy. They began with fire and water and went on from there. To the yang side went all that is bright, exciting, moving and warm. To the yin side went all that is dark, dull, still and cold. When they were done they had a list which placed every natural occurrence and state into one of the two camps. Here is a small example:
The yin-yang symbol (tai ji tu), again draws off of the day and night association of yin and yang. The story goes that the ancients plotted a graph made up of 6 concentrically larger rings. In the center they placed an 8 foot high stick and measured the shadow cast by the sun throughout the seasons. They then colored in where the shade landed and where there was none. When they looked at the graph from above, they ended up with a picture that extremely resembles the yin-yang without the two dots on either side. (For the whole story go to www.chinesefortunecalendar.com/yinyang.htm).
The addition of the smaller black and white circles are inserted into the opposite sides to further show the inseparable relationship between the two. A fun explanation of yin and yang is to think of a coin with yang being heads and yin being tails. They are two sides of the same thing. Now take that coin and spin it on a table and watch it make what looks like a globe. That is how close their relationship is.