In the Chinese zodiac, twelve animals are used to denote the year of a person's birth: rat, ox, tiger, rabbit, dragon, snake, horse, sheep, monkey, rooster, dog and pig. This is called a person's shengxiao (sheng means the year of birth, xiao means resemblance) or shuxiang. Since ancient times, Chinese have denominated years using combinations of 10 Heavenly Stems and 12 Earthly Branches to form sixty-year cycles. The 10 Heavenly Stems are: Jia, Yi, Bing, Ding, Wu, Ji, Geng, Xin, Ren and Gui. The 12 Earthly Branches are: Zi, Chou, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai. This year, February 9 is the first day of Yiyou, which is the twenty-second year of the sixty-year cycle.

As well as being associated with each year, the same twelve animals and Earthly Branches are assigned to each month and to a two-hour period of the day. Their origin is variously explained by a number of stories and theories.

One legend is that the order of animals is the result of squabbles that followed Emperor Xuanyuan's summoning them to be his imperial bodyguards. The rat tricked the cat out of going, and ever since they have been enemies. The rat also managed to drive the elephant away by climbing into his trunk. Of the other animals, the ox took the lead, but the rat jumped onto its back, hitching a ride into first place. The pig, busy complaining about this, came last. Since the tiger and dragon refused to accept the result, the Emperor compensated them with the titles "King of the Mountain" and "King of the Ocean," and placed them immediately after the rat and ox. But the rabbit would not accept this either, so raced and won against the dragon for fourth place. The dissatisfied dog bit the rabbit, and was punished with penultimate place. The other animals filled the other positions in the order in which they arrived.

The use of 12 animal symbols is not unique to the Hans in China. Many minority ethnic groups have their own series with minor differences. For example, Mongolians use tiger, rabbit, dragon, snake, horse, sheep, monkey, rooster, dog, pig, rat and ox; the Dai people use rat, ox, tiger, rabbit, dragon, snake, horse, sheep, monkey, rooster, dog and elephant; and the Li people use rooster, dog, pig, rat, ox, tiger, rabbit, dragon, snake, horse, sheep and monkey. Some believe that the Han inherited their twelve from northern tribes in ancient times. Another possibility is that exchange between different cultures cultivated the various sequences of twelve.

Shengxiao are held to be of great significance by many Chinese, and people tell numerous stories and follow rich and colorful customs associated with the Earthly Branches.