What's behind A, B, C and D?

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Followed by a, b, c and d: gypenoside, heptyl, heptyl and non-decyl.

A, B, C, D, E, self, G, Xin, people and ghosts are called heavenly stems.

Zi, Chou, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai are called earthly branches.

Heavenly stems and earthly branches, the main branch for short, originated from the observation of astronomical phenomena in ancient China. "A, B, C, D, E, Ji, G, Xin, Ren and Gui" are called ten heavenly stems, and "Zi, Ugly, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai" are called twelve earthly branches.

Heavenly stems and earthly branches constitutes the ancient calendar. Ten branches and twelve branches are matched in turn to form sixty basic units, which cooperate with each other in a fixed order to form the epoch method of 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.