Before a Qi Men Dun Jia (奇门遁甲 qí mén dùn jiǎ) plate says anything, it has to be built. The first fork in that build is the choice between yang dun yin dun (阳遁阴遁 yáng dùn yīn dùn) — the ascending and descending halves of the year. Everything downstream, the numbers, the direction the stars turn, the way the board is seeded, hangs off that one decision. This article walks the casting logic in principle. It does not produce a specific dated chart. The plate is deterministic and computed by an engine from the same rules that fix your Heavenly Stems and Earthly Branches, so we explain the method rather than invent positions.
The solstice split: yang dun and yin dun
Qi Men divides the solar year at the two solstices. Which half a moment falls in decides whether the chart is a yang dun or a yin dun plate.
Yang Dun 阳遁 — from the winter solstice (冬至 dōng zhì) up to the summer solstice (夏至 xià zhì). The formation numbers ascend, and the stars rotate forward.
Yin Dun 阴遁 — from the summer solstice up to the winter solstice. The numbers descend, and the stars rotate backward.
The image is the year breathing. From deep winter, yang qi is climbing back into the world, so the yang dun board counts upward and turns forward. From high summer, yin qi is gathering, so the yin dun board counts down and turns back. This is why yang dun yin dun is the very first thing a caster settles: it sets the direction of motion for the whole plate.
八字 · Four Pillars
Start with your own chart. Plot your Ba Zi free — exact solar-term accuracy, in your browser. The birth chart is where every reading begins.
The solstice tells you the half of the year. To place the pieces you need a finer address, and that comes from the solar term (节气 jié qì) and the three yuan.
Each solar term spans roughly fifteen days, and Qi Men splits that span into three yuan (三元 sān yuán): the upper, middle, and lower yuan (上元 shàng yuán, 中元 zhōng yuán, 下元 xià yuán), each five days long. The day’s position inside the term picks which yuan you are in.
Solar term plus yuan together select the ju number (局 jú), an index from one to nine. The ju number is the seed for the Earth Plate — it fixes how the Three Wonders and Six Yi (三奇六仪 sān qí liù yí) are laid across the Nine Palaces (九宫 jiǔ gōng). In a yang dun plate the ju numbers run in ascending sequence; in a yin dun plate they run descending. The full solar-term-to-ju table is standard and lives in the engine; it is not something to derive by hand in prose, and reconstructing it from memory is exactly the kind of error that produces a wrong board.
The chief and the duty gate
With the Earth Plate seeded, two moving symbols still need placing: the Chief and the Duty Gate. Both are driven by the hour.
Every hour belongs to a ten-day cycle whose leader is the xun shou (旬首 xún shǒu). That leader determines which of the Nine Stars acts as the Chief (值符 zhí fú) and which of the Eight Gates acts as the Duty Gate (值使 zhí shǐ). The Chief and Duty Gate then rotate to sit over the palace of the hour’s leading Yi. In a yang dun chart they move forward through the palaces; in a yin dun chart they move backward. This rotation is where the Heaven Plate comes to life over the fixed Earth Plate below it.
The Chief is the commander, the querent’s own standing on the board. The Duty Gate is where action and travel are read. Getting their placement right depends entirely on having called yang dun yin dun correctly at the start — a reversed rotation puts every later symbol in the wrong seat.
Where the schools genuinely differ
Two honest forks sit inside this casting process, and a working practice names them rather than papering over them.
The first is how the calendar’s untidiness is absorbed. The solar terms and the five-day yuan do not divide the year into clean equal pieces, so a rule is needed to reconcile them. The 拆补法 (chāi bǔ fǎ) and 置闰法 (zhì rùn fǎ) schools handle that reconciliation differently, and the 茅山 (máo shān) lineage differs again. They can hand you different ju numbers near the seams of a term, which means different plates.
The second is how the Heaven Plate stars and gates move. The 转盘 (zhuàn pán) rotating-plate school and the 飞盘 (fēi pán) flying-plate school use different motion for the stars and gates over the palaces. Both are legitimate. What matters is that a caster picks one and stays inside it, because mixing 转盘 and 飞盘 logic, or mixing 拆补 and 置闰 solar-term handling, produces a board that is internally inconsistent and therefore unreadable.
None of these choices are cosmic mysteries. They are calendar-arithmetic conventions, and the discipline is simply consistency: one solstice call, one solar-term rule, one motion school, applied end to end.
Why we describe the method, not a plate
You will notice this article never says “at such-and-such hour the Life Gate lands in the north.” That is deliberate. A Qi Men plate is computed, not recalled, in the same spirit that a Ba Zi (八字 bā zì) chart is computed from birth data rather than guessed. The yang dun yin dun split, the ju number, the three yuan, and the chief and duty rotation are the machinery; the finished board is the engine’s output. Explaining the machinery honestly is worth more than a fabricated position dressed up as insight.
If you want to see deterministic casting in action on a system you can check today, plot your Four Pillars first — start free with the Ba Zi calculator, then read what Qi Men Dun Jia is to see how the strategy board sits alongside the destiny chart.
