Heavenly Stems (天干) and Earthly Branches (地支)

The heavenly stems are ten characters representing celestial energy: jia (甲), yi (乙), bing (丙), ding (丁), wu (戊), ji (己), geng (庚), xin (辛), ren (壬), and gui (癸). Jia (甲) is straightforward qi like a great tree, enterprising in nature. Yi (乙) is soft and flexible like grass and flowers. Bing (丙) is hot and bright like the sun; ding (丁) is warm and subtle like candlelight. Wu (戊) is heavy qi like a mountain or vast earth; ji (己) is receptive like garden soil. Geng (庚) is solid qi like steel, resolute and decisive; xin (辛) is sharp and sensitive like a jewel. Ren (壬) is broad and receptive like the ocean; gui (癸) is clear and wise like dew.

The earthly branches are twelve characters representing terrestrial energy: rat (子), ox (丑), tiger (寅), rabbit (卯), dragon (辰), snake (巳), horse (午), goat (未), monkey (申), rooster (酉), dog (戌), and pig (亥). The "zodiac animals" we commonly refer to are the twelve earthly branches. The earthly branches are more complex than the heavenly stems because each contains one to three hidden heavenly stems. These are called hidden stems, and they are the key to understanding the deep properties of each branch. Additionally, the combination, clash, penalty, break, and harm relationships between earthly branches are very important in Saju interpretation.

Both heavenly stems and earthly branches have yin-yang and five-element attributes. The heavenly stems are paired two by two and attributed to one of the five elements. Jia (yang wood) and yi (yin wood); bing (yang fire) and ding (yin fire); wu (yang earth) and ji (yin earth); geng (yang metal) and xin (yin metal); ren (yang water) and gui (yin water). Yang represents large, active, and extroverted qualities, while yin represents small, receptive, and introverted qualities. The earthly branches similarly correspond to five elements: tiger and rabbit are wood; snake and horse are fire; monkey and rooster are metal; rat and pig are water. Dragon, dog, ox, and goat are earth, each with subtle differences based on their associated seasons.

When you sequentially pair the ten heavenly stems with the twelve earthly branches, you create 60 combinations: jia zi (甲子), yi chou (乙丑), bing yin (丙寅), and so on. This is the 60-year sexagenary cycle (Gan-Zhi), the basic unit of East Asian timekeeping. The 60-year cycle takes 60 years to complete, so at age 60, the year of your birth returns. This milestone is called the 60th birthday. Since each pillar in a Saju is one of the 60 combinations, there are theoretically 60 × 60 × 60 × 60 ≈ 12.96 million possible combinations. However, since the month pillar is dependent on the year pillar and the hour pillar is dependent on the day pillar, the actual number of valid combinations is smaller.

Each of the four pillars in Saju has a heavenly stem above and an earthly branch below. The heavenly stem represents the outer, manifest quality, while the earthly branch represents the inner, hidden quality. If the stem is "words and actions," the branch is "true heart and latent potential." In Saju interpretation, we first examine how the day master relates to the other seven characters. We analyze combinations and clashes between stems, combinations, clashes, penalties, breaks, and harms between branches, and the root relationships between stem and branch. For example, if the day master is jia (甲) wood and the branches contain tiger (寅) and rabbit (卯), the tree has strong roots. If the branches contain monkey (申) and rooster (酉) metal, the tree faces an axe. Reading the stem-branch relationship three-dimensionally is the foundation of Saju interpretation.