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

What Is Taiji?
Taiji is the original state of the cosmos when the universe was believed 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, which represents those that are upward, active, bright, expand, exocentric, masculine, and lively.

Everything in the world has both sides, which are in constant motion and mutual transformation.
The motions of Yin and Yang, then, 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 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 Yin Yang Symbol or the Taijitu.
Yin Yang Symbol or 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 movement of the sun.
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.
In the daytime, there are cloudy and rainy days, hence the black dot on the white side.
At night, there are bright lights of the moon and stars, which are represented by the white 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: 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)