About 3,000 years ago, there are some wise people who observe phenomena in Nature; making use of some tree branches - long and short ones, inventing symbols of Yin and Yang which represent a pair of differentiated nature. The symbol of Yin is --and Yang is -.

They understand that tree branches are tree branches no matter how it looks. So there developed another concept named Tai chi (meaning Large and Center), in which Yin and Yang are embraced. In the Supplementary note of I Ching there is a relevant section reading The Way (Tao) is One . Yin and One . Yang.

Tao (The Way) { Tai-Chi (One) : Yin · Yang

In which Tai Chi (One) can be referred to anything: as large as nature; as small as dust (or atom or even smaller). Anything can be regarded as a Tai chi, this also includes the ones that had been denoted as Yin and Yang earlier. And this elaborated to the Four Diagrams(Yin.Yin; Yin.Yang;Yang .Yin; and Yang.Yang) and the Eight Diagramsin which some natural phenomena or environment which affects the living of human being are matched with. The Eight factors are The Sky or The Sun; The Ground or The Earth; Earthquake; Wind; Water; Fire; Mountain; Pools & Ponds. The first four factors are considered large and far from the human control, whereas the latter four factors are relatively smaller and closer and they affects closely the everyday activities of human being.

Tai chi : Yin . Yangis the basic concept and the fundamental framework of  "I Ching" meaning The Book of Change.