It refers to the interdependence and mutual reinforcement between yin and yang. One is always a prerequisite or condition for the other. Neither yin nor yang can exist in isolation. For example, the upper is yin while the lower is yang. The upper does not exist without the lower, and vice versa. Meanwhile, yin and yang will generate, increase, and promote each other on the basis of their interdependence. For example, yin essence is the substantial foundation for the conversion and generation of yang qi in human body. The guard of yin essence in the interior part of the human body requires the consolidation of yang qi. Yin essence and yang qi are interdependent and promote each other.