Yin Yang

It is interesting to note that debates on yin yang continue in forums after more than two thousand years of their origin. That proves their popularity. When the terms yin yang came into used during the Warring States era they were meant to explain the effects of Qi (breath/energy) in the universe, heaven, on earth and man. Of equal interest is that both the Book of Changes (the Yi) and Tao Te Ching explain the effects of heaven and earth, the light and the dark on man, but made no specific references to yin yang.
Scholars of the Warring States, the Han or later dynasties probably ascribed the usage of yin and yang to Ancients to gain creditability and acceptance and to help explain the abstract meanings of heaven and earth, light and dark. With the advent of philosophical and religious Taoism, inner alchemy, fengshui, martial arts (qigong/taiqiquan/others) and traditional Chinese medicine (TCM), the original meanings of yin yang expanded to cover a wider scope of subjects especially those deemed to have a polarity as yin is taken to represent negative (-) or dark forces while yang represents positive (+) or light forces.
Consequently broken (dark) lines and unbroken (light) lines in trigrams and hexagrams of the Yi are read as yin and yang lines respectively. TCM doctors advocate a balance of both yin and yang to restore health while fengshui masters create yang or positive forces to improve wealth and/or health.
Meanwhile the debating continues.