Yin & Yang are seen as mutually dependent opposites; the essential duality of the Universe. Yin transforms into Yang as Night transforms into Day, as the cold of Winter transforms into the heat of Summer.

Yin is the 'dark side of the mountain, cool, passive, still. Yang is the 'bright side of the mountain, warm, active, open. Yin is the Feminine principal. Yang is the Masculine principal.

The five basic characteristics of Yin & Yang are
1.) that they are opposite to each other,
2.) that neither can exist without the other (interdependent),
3.) that each is constantly consuming the other (interconsuming),
4.) that they are in a constant state of transforming into each other (intertransforming) and
5.) that because each contains the other within it, each can be divided infinitely

In Chinese Medicine, an understanding of Yin & Yang is vital. If a condition is Yin in nature, that is understood to be a deficiency of some sort. If a condition is Yang in nature, that is understood to be an excess of some sort. Given the ebb and flow of Qi, a Yin condition can, and probably should become Yang in nature as the patient recovers. At the same time, a Yang condition, such as a Cold or the Flu, can and often does become Yin as the body is depleted. Needless to say there are numerous permutations of Yin & Yang in TCM diagnosis. Both are usually present and it is their dynamic interaction that is the basis for diagnosing and treating any disharmony.

Concepts of Yin & Yang permeate Oriental thinking at all levels. Spring and Summer are seen as the Yang times of the year. Time for rebirth, renewal, growth and activity. Autumn and Winter are seen as the Yin times of the year. Time for the earth to go to sleep. It is a time for introspection and meditation.

Each day is divided into Yang & Yin. The morning is seen as the Yang time of the day, while the afternoon is the Yin part of the day. The true Yin part of the 24 hour cycle is the night. The first part of the night is referred to as the Yang within Yin part, while closer to morning is referred to as the Yin within Yin part. The hour or two before dawn is the lowest ebb of the body. Dawn is considered the Yang within Yang part of the day, while the afternoon is the Yin within Yang, and so on.

Looking at the world in terms of Yin and Yang gives one a real sense of how dynamic and ever-changing is the Universe. No matter how you are feeling today, it will change. The ancients say one never steps into the same river twice; One never awakens to the same day twice.