Yin Yang and Taiji — Meaning, Origin, Symbol, Relationship, and Utilization
Yin Yang, which evolved out of Taiji, is a simple ancient Chinese philosophical idea widely applied in astrology, calendar, religion, traditional medicine, Feng Shui, and other traditions and customs.
Introduction to the Yin Yang Symbol Taijitu
What are the Flow and Relationships of Yin and Yang

What Is Taiji?
Taiji was the original state of the cosmos when the universe was believed to be undifferentiated and the oneness.
With the constant movement of Taiji, it evolves out of the Yin and Yang.
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Yin is about contracting energy, which is downward, passive, dark, accumulated, introverted, feminine, and quiet.
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Yang is about releasing energy, representing upward, active, bright, expand, exocentric, masculine, and lively.

Everything in the world has both sides: constant motion and mutual transformation.
The motions of Yin and Yang generate the Greater Yin, Lesser Yin, Greater Yang, and Lesser Yang.
They are the Four Images, whose transformations formed the Five Elements (Metal, Wood, Water, Fire, and Earth) or Five Phases theory.
Click to Read More About the Four Images, Five Elements Theory
Therefore, Taiji is considered the Supreme Ultimate, the origin of everything, and the Dao from Daoism (or Taoism) Philosophy.

Introduction to the Yin Yang Symbol Taijitu
Yin Yang Symbol Taijitu, or Tai Chi Diagram, had different origins and versions in history.
Yin Yang and Taiji in Cosmology
As a cosmological term, the Yin Yang Symbol or Taijitu is the orbit of the sun's movement in a year.
In the Chinese Calendar, the 24 Solar Terms correspond to the sun's movement.
The lengths of the shadow of the ancient sundial on noontime of each Solar Term day formed the S-shaped curve in the middle.

The two colors represent day and night.
There are cloudy and rainy days in the daytime, hence the black dot on the white side.
At night, there are bright lights of the moon and stars, represented by the black dot on the black side.

Yin Yang and Taiji in Philosophy
As a philosophical term of both Taoism and Confucianism, the symbol perfectly represents the duality theory, relationship, and principle of Yin and Yang.
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Round Shape: the wholeness of the universe, intact and eternal circle, and Great Unity.
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Black and White Sides: equality of Yin and Yang, everything has two sides, the relationship of opposition and unification.
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Curve in the Middle: everything is in motion, hence can convert into the other side and situation, and with complementary forces;
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Dots on Opposite Colors' Side: nothing is absolute nor still; there's always some Yin on the Yang side, and vice versa.

Gold and Gem Decorations of the Ming Dynasty (1368 — 1644) — Hubei Museum (Photo by Dongmaiying)
Yin and Yang Meaning
Briefly speaking, Yin and Yang is a duality concept.
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Heaven is Yang, earth is Yin;
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Sun is Yang, moon is Yin;
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Brightness is Yang, darkness is Yin;
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Daytime is Yang, night is Yin;
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Thunder is Yang, rain and snow are Yin;
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Hot is Yang, cold is Yin;
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Spring and summer are Yang, autumn and winter are Yin;

Creative Taijitu Presents Seasons, By Feifei Fiona.
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Man is Yang, woman is Yin;
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Emperor is Yang, official is Yin;
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Movement is Yang, stillness is Yin;
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Odd number is Yang, even number is Yin;
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Up is Yang, down is Yin;
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Left is Yang, right is Yin;
To sum up, Yin and Yang can be concluded as two sides of anything.

Bricks of the Han Dynasty (202 BC — 220 AD) of Deities Flying Dragons Shapes — The Art Institute of Chicago (Photo by Dongmaiying)
What Are the Flow and Relationships of Yin and Yang
Everything is in motion, hence the constant flow between Yin and Yang.
And the flow leads to four relationships of the Yin and Yang:
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Opposition
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Interdependence
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Wax and Wane
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Transform

What Is the Yin-Yang Balance?
Balance of Yin Yang is a relatively stable status among the constant movement of Yin and Yang, which is believed to be the best situation.
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In cosmology, it symbolizes a natural world that all species coexist in harmony;
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In the climate field, it represents a stable amount of rain and sunshine and good weather;
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In traditional Chinese medicine, it means a healthy body;
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In romantic relationships, it is the key to equality and harmony.

Dragon and Phoenix Pattern Porcelain of the Qing Dynasty (1636 — 1912) — The Asian Art Museum of San Francisco (Photo by Dongmaiying)