The Yin-Yang School 阴阳派

The Yin-Yang School
A category of Neidan (internal alchemy) techniques, also known as the Dual-Cultivation School.

The Can Tong Qi (The Unity of the Three) states: "All things lack Yin and Yang, which goes against nature and deviates from the origin." Alchemists believe that just as all things in the world require the combination of Yin and Yang to form elixirs, the practice of internal alchemy in the human body also requires the dual cultivation of men and women, with the coordination of Yin and Yang, to form the elixir. Women are externally Yin but internally Yang, corresponding to the Kan trigram (☵); men are externally Yang but internally Yin, corresponding to the Li trigram (☲).

Practitioners of Neidan use the technique of Yin-Yang grafting to extract the innate true Yang from women (known as "gold in water") and replenish it into the position of the Yin line in the middle of the man's Li trigram. This is called "taking Kan to fill Li," which is the basic technique of the Yin-Yang School's alchemy methods.