The familiar figure, two interlocking teardrops, each carrying a dot of the other's colour, in a circle, is late. It is usually attributed to Zhao Huiqian in the fourteenth century, and is sometimes called the xiantian taiji tu, the diagram of the supreme polarity prior to heaven. The attribution is the standard one but it is a matter of textual scholarship rather than a documented signature.
The design is remarkably good, which explains its survival. Three claims of the underlying philosophy are stated by the drawing itself and not by any caption: the two halves are equal in area, so neither dominates; the boundary between them is a curve rather than a line, so each is continuously becoming the other rather than confronting it; and each contains a spot of its opposite, so nothing is ever purely one thing. A person who has only seen the figure has already absorbed the substance of the doctrine.
Its adoption as the general emblem of Taoism is later still and is partly a Western convenience, in the way that a cross serves for Christianity. Chinese religious practice did not depend on it, and it also circulates in Confucian, medical and divinatory contexts that are not Taoist at all.
Its modern career is extraordinary and largely detached from any of this. It appears on the flag of South Korea in a modified form, on unit insignia, on packaging, and as a general shorthand for balance. Along the way the relational and processual sense of yin and yang has commonly been flattened into a static opposition of male and female or good and evil, which is close to the reverse of what the classical texts argue.