from A Short History of Chinese Philosophy
Through these two primary principles, all phenomena of the universe are produced.
Later, the theory of Yin Yang came to be connected primarily with 易经(Yì Jīnɡ) - the Book of Change. The original corpus of this book consists of what as known as 八卦(bá ɡuà) - the eight trigrams, each made up of combinations of three divided or undivided lines, as follows: ☰, ☱, ☲, ☳, ☴, ☵, ☶, ☷. By combining any two of these trigrams with one another into diagrams of six lines each, a total of sixty-four hexagrams.
In the trigrams, the undivided lines symbolize the Yang principle, and the divided lines the Yin principle. The trigrams 乾(Qián, heaven) ☰ and 坤(Kūn, earth) ☷, beiing made up entirely of undivided and divided lines respectively, are the symbols of par excellence of the Yang and Yin, while the remaining six trigrams are supposedly produced through the intercourse of these primary two.Thus 乾(Qián) ☰ and 坤(Kūn) ☷ are father and mother, while the other six are sons and daughters.
This process of combination or intercourse between 乾(Qián) and 坤(Kūn) which results in the production of the remaining six trigrams, is a graphic symbolization of the process of intercourse between the Yin and the Yang, whereby all things in the world are produced. That the world of things is produced through such intercourse of the Yin and the Yang, is similar to the fact that living beings are produced through the intercourse of the male and female. The Yang is the principle that "gives beginning" to things, the Yin is that which "completes" them.
In the religion of the primitive Chinese, it was possible to conceive of a father god and a mother goddess who actually gave birth to the world of things. In the Yin-Yang philosophy, however, such anthropomorphic concepts were replaced by, or interpreted in terms of, the Yin and Yang principles, which though analogous to the female and male of living beings, were nevertheless conceived of as completely impersonal natural forces. Everything in the universe come from the two forces. The two forces are not total opposites, they are relative to each other. The way that Yin and Yang work, is 道(Dào) - Tao.