This set of twelve animals is symbolic of the twelve-year cycle in the Chinese calendar and the twelve two-hour periods in the day. The set also represents the twelve divisions of the sky as well as the directions of the compass. Correlated with certain philosophical ideas popular in the Han period—the principles of Yin, Yang, and the Five Elements—these animals are believed to possess the power of regeneration when put in the following order: rat, ox, tiger, rabbit, dragon, snake, horse, goat, monkey, cock, dog, and pig.The association of the cycle of twelve divisions with a set of animals occurred by the third century B.C. There is no evidence of the pictorial representation of the set before the sixth century, when images of the animals began to appear on the backs of bronze mirrors and on the walls of tomb chambers. During the Tang dynasty (618–907), when burial customs were most elaborate, sets of pottery figures in official robes with heads of the calendrical animals were commonly used in burials. It is presumably the association of the set of animals with the power of regeneration that prompted their use in funerary rituals.