information:classification
Stem-branches

The 60 stem-branches have been used to mark the date since the Shang dynasty (1600 BCE – 1046 BCE). Astrologers knew that the orbital period of Jupiter is about 12×361 = 4332 days, which they divided period into 12 years (歲; 岁; suì) of 361 days each. The stem-branches system solved the era system's problem of unequal reign lengths.