The primary meaning of yin and yang is the orientation of things in relation to the sun, with yang meaning the sunny side and yin the shady side. There are two extended meanings: 1) two opposite kinds of qi (气) in nature; and 2) two basic contrary forces or qualities that coexist, thus the active, hot, upward, outward, bright, forward, and strong are yang, while the passive, cold, downward, inward, dark, backward, and weak are yin. The interaction between yin and yang, or yin qi and yang qi, determines the formation and existence of all things. The theory of yin and yang later became the basis for ancient Chinese to explain and understand the universe and everything in it, social order, and human relations. For example, heaven is yang and earth is yin, ruler is yang and subordinates are yin, husband is yang and wife is yin, noble is yang and ignoble is yin, leading is yang and following is yin.