Heavenly stems and earthly branches, referred to as the "main branch" for short, originated from the observation of astronomical phenomena in Archaean China. "A, B, C, D, E, B, G, A, B, C, B" is called ten-day work;
"Zi, Ugliness, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai" are collectively called twelve branches.