If Yin and Yang are black and white, the five elements are shades of grey.

When seeking to understand and describe the natural world around them the ancient Chinese developed a system of classification based upon natural elements that they were able to observe in nature. 

The system of the five elements is composed of fire, earth, metal, water, and wood.   The five elements is sometimes called the five phases because rather than being thought of as fixed classifications this system describes these elements as being interconnected and interchanging.  As qualities that morph and flow into one another.

The concrete example of seasons is the best way to comprehend this system.  Winter is known to have the quality of water.  Things tend to be still and cold.  The term “deep” of winter describes the abyss like quality of water and wintertime. 

Water in the chinese five element cycle transforms to the element of wood which is the element ascribed to springtime.  The quality of springtime shares the quality of a newly growing plant(composed of wood!).  The energy is growing, yearning upward, the color of spring is green which is the color of the wood element. 

Wood in the five element system gives rise to the fire element.  Summer is the season of fire: it is hot and active.  People are moving and connecting rapidly much as a fire consumes and overtakes its fuel source.  

The chinese have a concept of a “fifth season”  known as late summer.  More on that concept here http://acupunctureology.com/late-summer-and-the-earth-element/

The concept of Metal as being part of an elemental system often confuses those new to Oriental Medicine.  Many folks have a peripheral awareness of the ancient greek system of the elements namely: water, fire, and air.  The ancient chinese observed that metal in the form of Iron ore came from the earth in the form of stones.  This metal ore was very much a part of their day to day existence so it was natural for them to include this “element” as a part of their worldview and cosmology(the study of the universe)

Metal has the quality of being of a fixed unyielding nature.  Metal contracts when it gets cold much like the season of Autumn does.  The chinese saw this as perfectly representing the contracting unyielding nature of fall.

Now how does this apply do the practice of medicine.   Well, this system of the five elements became highly developed and applied to a broad array of topics in chinese culture including the practice of medicine.

 Once these qualities are understood on a basic level they can be applied to anything and everything including the body and its physiology.  A persons appearance, mannerisms, skin color, voice tone, odor, and temperament can all be classified according to the five elements.  There is one school of acupuncture known as the five element school which focuses on these classifications solely.