The Yin and Yang symbol, known as the taijitu, depicts the balance between two supposed opposites. But are these elements really opposite? Look closer and you'll see that a portion of each opposite element appears in the other element.  This is because nothing is absolute. You'll always find some Yin within Yang and sone Yang within Yin.

YIN and YANG describe change.

Change is certain, a basic law of nature you can count on.

Originally YIN and YANG referred to the sides of a mountain.
In the morning, one side is in shade, and the other in sunlight.
Later in the day, the sides have changed. 
The light side has become dark and dark side has become light.

Nature is like this, forever changing, undulating In time.

Under the influence of time, YANG always turns to YIN, and YIN predictably becomes YANG.

Examples of YIN and YANG in Nature

Hot is Yang, cold is Yin

Light is Yang, Dark is Yin

Daytime is Yang, Nighttime is Yin

Activity is Yang, rest is Yin

Function is Yang, structure is Yin

Male is Yang, female is Yin

Dryness is Yang, Dampness is Yin

The Sun is Yang, the Mood is Yin

Examples of YIN and YANG in the Body

Upper body is Yang, Lower body is Yin.

Front of the body is Yin, Back of the body is Yang.

Surface of the body is Yang, interior is Yin

Acute diseases are yang, Chronic diseases are Yin

Excess conditions are Yang, Deficient conditions are Yin.

The major organs (lung, heart, spleen, liver, kidney) are the Yin Organs.

The minor organs (colon, small intestine, stomach, gall bladder, and urinary bladder) are the Yang Organs.

The Blood is Yin, the Qi is Yang.

YIN and YANG Help Us Restore Harmony

Practitioners of Oriental medicine, use the idea of Yin and Yang to describe harmony and balance.

We see the body and its disharmonies in changing shades of YIN and YANG.  Extreme behaviors, excessive emotions, unchanging habits, extreme diets, and rigid attitudes un-permitting of change, can all result in lopsided Yin or Yang. Where Yin or Yang y dominate one another.  This imbalance will invariably result in physical and/ or mental distress. 

Studying Yin and Yang helps us to assess balance and harmony and understand the root of many diseases.