Chinese acupuncture based upon stems branches is one type of acupuncture that pays specific attention to the time elements, it is an important type of traditional Chinese acupuncture. In Nei Jing, it is maintained that man corresponds to heaven and earth, both natural phenomena in the universe and energy circulation in the human body can be calculated by reference to the pattern of stems and branches with stems representing heaven and branches representing earth. This is why stems are also called the heavenly stems and branches are called the earthly branches. There are two basic words used to represent two periods of time: namely, Zi (or Tzu in older pinyin) stands for the first period of time daily which is 11 p.m. to 1 a.m., and Wu stands for the 7th period of time which is 11 a.m. to 1 p.m.. A Chinese medical classic says, "Yang begins prior to the Zi period and ends after the Wu period. Yin begins after the Wu period and ends before the Zi period; yin and yang are in abundance and in decline according to different periods of time; they begin and end without a stop."