The use of 12-year animal cycles for recording the years in China dates back to 100 A.D. Each year is symbolized by an animal and the 12 animals are mouse, ox, tiger, rabbit, dragon, snake, horse, goat, monkey, rooster, dog and pig.

     The 12-year animal cycle corresponds to the 12 earthly branches (Figure 3). In a Jia-Zi cycle, those years with the same earthly branch are symbolized by the same animal. For example, the earthly branch "Shen" is symbolized by monkey so Ren-Shen, Jia-Shen, Bing-Shen, Wu-Shen and Geng-Shen are all years of the monkey. The Gan-Zhi years that correspond to other animals can be determined similarly.

Figure 3 The 12 earthly branches and the 12 animals.