天干 and 地支
Stems and Branches
The oldest layer of Chinese timekeeping, and the ground the zodiac stands on.
The two sets
Ten stems, twelve branches
The ten Heavenly Stems, 天干, are 甲 乙 丙 丁 戊 己 庚 辛 壬 癸, first attested around 1250 BCE as the ten days of a ritual week; consecutive pairs share a phase, one yang and one yin. The twelve Earthly Branches, 地支, are 子 丑 寅 卯 辰 巳 午 未 申 酉 戌 亥; each carries a phase and a polarity, and the animal names were attached later.
The sixty-year cycle
Why sixty and not a hundred twenty
Each term of the cycle, 六十甲子, is one stem with one branch, advancing together. The stems repeat every ten and the branches every twelve, so the pattern returns after sixty. Of a hundred twenty conceivable pairs only sixty occur, because a yang stem pairs only with a yang branch and a yin stem only with a yin branch. The cycle is documented on Shang oracle bones, one of the oldest notations in continuous use.
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