Gapja, Eulchuk, Byeongin… what exactly are these unfamiliar syllables that show up in every conversation about saju (Korean four-pillars astrology)?
The heavenly stems (cheongan) and earthly branches (jiji) are the alphabet of the language called saju. Ten stems carry the energy of the sky, twelve branches carry the energy of the earth, and combinations of these 22 characters build the eight characters of a saju chart. If phrases like “Gap-wood day master” or “the year of Byeong-o” have felt like code, they unlock the moment you learn what these characters are. There are fewer of them than the 26 letters of the alphabet, so no pressure.
The Heavenly Stems: Ten Energies of the Sky
The heavenly stems are the five elements split into yin and yang, making ten characters. They carry visible temperament, aspiration, and the texture of the mind.
- Gap (甲) the great tree, a leader’s grain that grows straight up
- Eul (乙) flowers and vines, a flexible grain that winds and climbs
- Byeong (丙) the sun, an expressive grain that shines on everything
- Jeong (丁) the candle, a delicate grain that warms what is near
- Mu (戊) the mountain, an unshakable grain of steadiness
- Gi (己) the field, a practical grain that holds and nurtures
- Gyeong (庚) raw iron, a decisive grain that cuts and pushes through
- Sin (辛) the jewel, a polished, incisive grain
- Im (壬) the ocean, a broad-flowing grain of embrace and strategy
- Gye (癸) misty rain, a permeating grain of sensitivity
The stem of the day you were born is your day master, in other words, “you.” All that content about “what a Gap-wood day master is like” is really about these ten characters.
The Earthly Branches: Twelve Stages of the Earth
The earthly branches are the twelve characters of seasons and hours: Ja (rat), Chuk (ox), In (tiger), Myo (rabbit), Jin (dragon), Sa (snake), O (horse), Mi (sheep), Sin (monkey), Yu (rooster), Sul (dog), Hae (pig). Yes, the zodiac animals are the earthly branches. The branch of your birth year is your zodiac sign.
If the stems are the texture of the mind, the branches are the stage of reality. The seasons (In-Myo-Jin = spring, Sa-O-Mi = summer, Sin-Yu-Sul = autumn, Hae-Ja-Chuk = winter) and the hours of the day live in the branches, and in saju interpretation the chemistry of relationships (combinations and clashes) happens mostly among the branches. Special stars (sinsal) like Dohwa, Yeokma, and Hwagae are also combinations of branches.
The 60 Gapja: Why a 60th Birthday Is a Full Cycle
Pair the ten stems with the twelve branches in order and you get Gapja, Eulchuk, Byeongin…, and after 60 combinations, the least common multiple of 10 and 12, the cycle returns to the beginning. That is the 60 gapja.
That is why every year gets a name. 2026 is the year of Byeong-o, the year of Byeong (the sun) and O (the horse), which is why it is called the “year of the red horse.” It is also why hwangap (還甲), the 61st birthday, is celebrated: the gapja of your birth year finally comes back around. And the 60 gapja cycles not only through years but through months, days, and hours. The daily iljin runs on a 60-day cycle for the same reason.
Frequently Asked Questions (FAQ)
Q. Which matters more, the stems or the branches? Neither, their roles are different. The stems show visible temperament and aspiration; the branches show real-world conditions and the stage of timing. Saju interpretation always reads the two layered together. For a beginner, though, the right order is to start with your own day master (a stem).
Q. Gap and Eul are both wood, what is the difference? Yin and yang. If Gap wood is a pine shooting straight up, Eul wood is a vine winding its way up a rock. The same energy of growth splits into a head-on type (Gap) and a flexible, roundabout type (Eul). All ten characters pair up this way.
Q. What does it mean that stems are hidden inside the branches? That is an advanced concept called jijanggan: each branch stores several stem energies inside it. At the beginner stage, it is enough to know that “branches are more complex than they look.”
These 22 characters are both the whole of saju and its starting point. Once you know them, phrases like “the year of Byeong-o” or “a Gyeong-metal day master” stop being code and start reading as descriptions of seasons and temperament.
You can check right away which stems and branches make up your own eight characters in Sajuping (sajuping.ai), the AI saju consultation app built on the manseryeok (the saju calendar). In the next post, we will sort out how these characters relate to one another: the ten gods (sipseong).