Yin and yang are terms used to describe relative opposite qualities or manifestations of Qi. If yin is form, then yang is function. If yin is material, then yang is immaterial. Yin refers to aspects or manifestations of Qi that are relatively material, substantial, condensing, solid, heavy, descending, cold, moist, cooling, dark, passive and quiescent. Yang refers to aspects or manifestations of Qi that are relatively immaterial, amorphous, expanding, hollow, light, ascending, hot, dry, warming, bright, aggressive, and active.