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Heavenly Stems and Earthly Branches

        Heavenly stems and earthly branches are collectively known as Stem-Branch or Gan-Zhi.  The 10 heavenly stems are Jia, Yi, Bing, Ding, Wu, Ji, Geng, Xin, Ren and Gui.  The 12 earthly branches are Zi, Chou, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai.  Each heavenly stem is paired with an earthly branch to form the Gan-Zhi sexagenary cycle that starts with Jia-Zi (Table 2). 

         Table 2             The Gan-Zhi sexagenary cycle.

        The use of Gan-Zhi to record the years, months, days and hours all started in different periods in history.  The Gan-Zhi years and days have been used for two to three thousand years.  In ancient time, the months and the hours (the hours are called shichen) were named after the earthly branches only (Figure 1 and Table 3).   Through the years, people started to make stems of the months after the stem of the year (Table 4), and stems of the hours after the stem of the day (Table 5).  The Gan-Zhi's of year, month, day and hour together form the Eight Characters.

         Table 3            The relationship between the 12 shichen and the present time system.