Heavenly Stems & Earthly Branches
Establishments & Uses of Heavenly Stems & Earthly Branches
Initially ten stems were created to record the days, for example - 1st day (Jia - H1), 2nd day (Yi - H2), 3rd day (Bing - H3), 4th day (Ding - H4), 5th day (Wu - H5), 6th day (Ji - H6), 7th day (Geng - H7), 8th day (Xin - H8), 9th day (Ren - H9) and 10th day (Gui - H10).
The twelve branches were created to record the months, for example - 1st month (Zi - E1), 2nd month (Chou - E2), 3rd month (Yin - E3), 4th month (Mao - E4), 5th month (Chen - E5), 6th month (Si - E6), 7th month (Wu - E7), 8th month (Wei - E8), 9th month (Shen - E9), 10th month (You - E10), 11th month (Xu - E11) and 12th month (Hai - E12).
There was one problem using the above system. The ten heavenly stems get repeated 3 times in a month. This makes recordings confusing as there exists three Jia days in a Zi month and so on. It would be far more complicated when we involves the time, day, month and year.
The 60 Combinations
There are 10 heavenly stem and 12 earthly branches. Each stem is paired with an earthly branch, forming a 'pillar'. It is important to note that only odd-numbered stems are paired with odd-numbered branches, and even-numbered stems with even-numbered branches. For example, the first heavenly stem can be paired with the first earthly branch, the third or the fifth one etc, but cannot be paired with the second or the fourth branch etc.
Henceforth, instead of 120 combinations, we get 60.