24 Mountains 二十四山 Glossary
The 24 directional names that replace a bare 360° scale on the Chinese compass: eight heavenly stems, twelve earthly branches and four corner trigrams, each occupying exactly 15°.
Èrshísì shān · 二十四山 (simplified) · 二十四山 (traditional)
Source credibility (S/A/B/C/X): A Rule computability (A/B/C): A
What it is
A western compass says "north"; a Chinese one says which mountain （山） the needle points to. The circle is divided into 24 sectors: the eight stems 甲乙丙丁庚辛壬癸 (the four others — 戊己 — are omitted), the twelve branches 子丑寅卯辰巳午未申酉戌亥, and the four corner trigrams 乾坤艮巽 as the diagonal sectors. Each mountain spans exactly 15°. With 0° at true north, the Zi mountain runs 352.5°–7.5°, so the Zi–Wu axis straddles due north–south; Wu runs 172.5°–187.5°.
Two structural facts make the system computable. First, the facing is always the sitting plus 180°: a Zi sitting is a Wu facing. Second, within each trigram palace the three mountains are classed as elemental dragons （三元龍） — heaven, earth and human （天元／地元／人元龍） — and each class carries a fixed yin or yang that decides flying direction in the Xuan Kong Flying Star chart. The same 24-mountain skeleton also hosts the fenjin subdivisions (five 3° slots per mountain) that define the lower-trigram / replacement boundary.
School differences
The mountain table itself is common ground; what differs is which ring of it each lineage uses. San He practice reads sand and water on the rotated middle and sewing needles (each a 7.5°-shifted copy of the 24-mountain ring), while Xuan Kong takes its sitting and facing exclusively from the correct needle — see Luopan. The project's School Dispute Register keeps the needle-attribution disputes alongside the chart disputes.
How this engine computes it
- The full 24-mountain degree table (start / centre / end / lower-trigram zone / ti-gua zones / elemental-dragon class for every mountain) is A-grade data in the kernel, verified line by line during collation.
- Sitting–facing conversion is arithmetic: facing = sitting + 180° (mod 360°); the engine labels which of the pair is the back (sitting) and which the front (facing) when a reading is given.
- All boundary-line degree values ending in 2.5 or 7.5 are asserted by property tests, so a corrupted table cannot pass unnoticed.
- Compass input is expressed in absolute degrees (never "clockwise/counter-clockwise from X", where sources contradict each other) and mapped to mountains, with the uncertainty of the reading carried through to the chart-type judgement.
Related entries
Luopan · Xia Gua / Ti Gua · Xuan Kong Flying Star · Sansha
Back to the glossary index.