Heavenly stems and earthly branches, referred to as "dry branch". In the ancient calendar of China, A, B, C, D, E, Ji, G, Xin, Ren and Gui were called "ten heavenly stems", while Zi, Ugly, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai were called "twelve earthly branches". Ten branches and twelve branches are matched in turn to form sixty basic units, and they cooperate with each other in a fixed order to form branches and branches. Judging from the Oracle Bone Inscriptions unearthed in Yin Ruins, in ancient China, the trunk and branch were mainly used to record the days, as well as the months, years and hours.