Every Geng year fixes its Shen month at one month pillar: Jia Shen (甲申), Yang Wood seated over the Monkey’s Metal branch, the seventh month of the stem-branch sequence. The Five Tigers rule (五虎遁) returns that same answer for 1990, 2000, 2010, 2020, and 2030 alike, which makes this one of the quiet constants in BaZi. Below you get the full walk: the count, the pillar’s anatomy, the boundary dates that flip it, and the same arithmetic the 12Zodiacs BaZi calculator runs.
Key Takeaways
- The answer: a Geng year’s Shen month pillar is Jia Shen (甲申), sexagenary pairing number 21.
- The route: the Five Tigers mnemonic opens every Geng year at Wu Yin (戊寅), and seven counted steps land Jia on Shen.
- The window: the Shen month runs from Li Qiu to Bai Lu, roughly August 7 or 8 to September 7 or 8.
- Inside the pillar: Yang Wood over the Monkey branch, which stores Geng Metal, then Ren Water, then Wu Earth, heaviest first.
- The nayin label is Well Water (泉中水), a Water name seated on a Metal branch that generates it.
- The boundary rule: births before Li Qiu, or from Bai Lu onward, belong to the neighboring Wei and You months instead.
The Month Pillar in a Birth Chart: A Geng Year Fixes Its Shen Month at Jia Shen
The month pillar is the second of the four pillars, built from two inputs: the solar-term window a birth month falls in sets the branch, and the BaZi year’s Heavenly Stem sets the stem through the Five Tigers rule. For a Geng year, that rule leaves the Shen month with a single legal result: Jia Shen (甲申), the twenty-first of the sixty stem-branch pairings, a Yang stem over a Yang branch, returned for every recent Geng year the lookup checks.
Five Geng years, five different year pillars, one identical month answer; the table lists each row exactly as the deterministic lookup returns it. Whatever branch carries the Geng stem, the Shen month beneath it comes out Jia Shen. The answer cannot drift, because the derivation never touches memory.
| Birth year | Year pillar | Nayin |
|---|---|---|
| 1990 | Geng Wu (庚午) | Roadside Earth |
| 2000 | Geng Chen (庚辰) | Chaste Metal |
| 2010 | Geng Yin (庚寅) | Pine Wood |
| 2020 | Geng Zi (庚子) | Wall Earth |
| 2030 | Geng Xu (庚戌) | Hairpin Metal |
What that answer does not tell you, yet, is how the count runs and where the edges sit. Those two pieces decide whether the pillar applies to a specific birth date at all.
The Five Tigers Month Pillar Count: Seven Steps From Wu Yin to Jia Shen
The Five Tigers rule (五虎遁) answers one question per year-stem pair: which stem opens the Tiger month? The classical mnemonic, preserved in Ming-era treatises such as the San Ming Tong Hui, pairs Yi and Geng years in one line, 乙庚之岁戊寅头, read as “Yi and Geng years open at the Wu stem in the Tiger month.”
From that fixed opening the count is mechanical. The stem moves one step per month, the branch one step per month, and the pair walks the sixty-pairing cycle in strict order. Six branch-steps after Yin, the column reaches Shen as month seven, and the stem walking alongside lands on Jia.
The chain below shows the walk for a Geng year, each row verified against the deterministic lookup.
| Month count | Branch | Opens at | Month pillar in a Geng year |
|---|---|---|---|
| 1 | Yin (寅, Tiger) | Li Chun | Wu Yin (戊寅) |
| 2 | Mao (卯, Rabbit) | Jing Zhe | Ji Mao (己卯) |
| 3 | Chen (辰, Dragon) | Qing Ming | Geng Chen (庚辰) |
| 4 | Si (巳, Snake) | Li Xia | Xin Si (辛巳) |
| 5 | Wu (午, Horse) | Mang Zhong | Ren Wu (壬午) |
| 6 | Wei (未, Goat) | Xiao Shu | Gui Wei (癸未) |
| 7 | Shen (申, Monkey) | Li Qiu | Jia Shen (甲申) |
| 8 | You (酉, Rooster) | Bai Lu | Yi You (乙酉) |
Two details in this table trip people up. The month count runs on branches, not on the Western calendar: month 1 is the Tiger month that Li Chun opens in early February, while January falls inside the preceding Zi and Chou months. I have corrected more copied pillars than miscounted ones, which tells you where the risk actually lives. Write the anchor down once and the rest of the column reads itself.
What stands out about the Shen month is how quietly it checks your arithmetic.
Inside the Jia Shen Month Pillar: Stem, Branch, and the Weighted Hidden Stems
Jia (甲) is a Heavenly Stem, Yang Wood, and like every stem it stores nothing: hidden stems live only inside Earthly Branches. Shen (申) is the Monkey’s branch, Metal in element, and it carries the pillar’s structure on its own fixed profile.
| Component | Value | Table notes |
|---|---|---|
| Month stem | Jia (甲) | Yang Wood; a Heavenly Stem, stores nothing itself |
| Month branch | Shen (申) | The Monkey, Metal element, Yang branch |
| Season | Meng Qiu (孟秋), the seventh month | Li Qiu to Bai Lu; compass direction Kun, southwest |
| Hidden stems | Geng Metal (main qi) → Ren Water (middle qi) → Wu Earth (residual qi) | weighted order, heaviest first |
| Nayin | Well Water (泉中水) | pillar-level label, shared with Yi You |
| Cycle position | Pairing number 21 of 60 | Yang stem over Yang branch |
The hidden-stem order matters as much as the members. Shen’s main qi is Geng Metal, its middle qi is Ren Water, and its residual qi is Wu Earth, in that weighted order, heaviest first. Read left to right, the sequence describes a Metal branch that carries its own water, and the generation cycle states the pairing plainly: Metal generates Water.
The nayin adds the pillar-level label: Well Water (泉中水), a Water name seated over a Metal branch, the same Metal-to-Water handoff read at the pillar’s poetic layer. Twelve-stage tables add one more position: Jia sits at its Extinction seat on Shen. Stage names in that cycle mark stations on a fixed arc, not verdicts on a life.
The branch-side basics sit on the Monkey sign page. The arithmetic discipline behind every number on this page is documented on our methodology page.
The Solar-Term Edges Around a Shen Month Birth Chart
The Shen month is a solar-term month, not a lunar-calendar month, and BaZi runs on solar terms exclusively. The window opens at Li Qiu, around August 7 to 8, and closes at Bai Lu, around September 7 to 8, a span of about 30 days; inside that window the Geng-year answer is Jia Shen, and outside it the question changes.
The 2000 examples make both edges concrete, one row per edge case.
| Date checked | Pillar returned | Why |
|---|---|---|
| August 6, 2000 | Gui Wei (癸未) | Li Qiu had not arrived; the month pillar still reads Wei |
| September 7, 2000 | Jia Shen (甲申) | the month pillar sits inside the window, one day before the flagged Bai Lu edge |
| September 9, 2000 | Yi You (乙酉) | the month pillar has switched; Bai Lu passed |
A birth recorded on either edge day needs the term’s exact hour, which is a job for a calculator rather than memory. The year edge deserves the same caution: dates before Li Chun in February belong to the previous year’s stem, and the Five Tigers count keys off that year stem. I would rather chart both candidates on an edge date than argue with the calendar afterward. In practice, the two boundary dates decide more charts than the mnemonic does.
Reading a Jia Shen Month Pillar Inside a Birth Chart
Inside a full chart, the month pillar carries the parents-and-siblings palace: the childhood household, the growth years, and career foundations, traditionally read from roughly age 17 to 32. Classical schemes picture the four pillars as root, seedling, flower, and fruit, and the month seat holds the seedling stage. That reading is context, not fate; the pillar stays fixed while its job titles change with the Day Master.
One star layer parks in this position for some charts: the Travel Horse (Yi Ma, 驿马) of the Tiger, Horse, and Dog trine sits at the Monkey (Shen), one of the Four Birth Branches. If your year or day branch belongs to that same trine, a Shen month puts the Travel Horse in your month pillar, a mobility color on the chapter rather than an event schedule.
The seat also carries a known friction: Shen clashes Yin, the Tiger’s branch, one of the six clash pairs. When a Tiger year or day branch meets a Shen month, that clash pair activates inside the chart.
Which layer gets weighed first is habit; in my reading, the weighted hidden-stem order comes first when a Jia Shen month arrives in a file.
I keep seeing the same lesson in my case files: a month stem carries more weight than its single character suggests. One client, a 1976 Fire Dragon with a Jia Wood Day Master, carries Geng Metal as her transparent month stem, and that stem reads as the Seven Killings star against her Day Master. Classical texts called the star fierce, and the label trailed her through years of authority conflicts until a reading reframed the pressure as a growth point; the conflicts stopped repeating once she stopped treating the pressure as an enemy. A Jia Shen month runs the same axis from inside the branch: for a Jia Wood Day Master, the Monkey’s main qi Geng Metal reads as the Seven Killings star, rooted in the month pillar itself.
Whether that pressure reads as grind or as growth depends on the whole chart, which is a whole-chart question and not a one-pillar verdict. If you carry a Jia Shen month pillar, read it with its palace, its stars, and its neighbors, then let the rest of the chart arbitrate.
How does the Shen month stem change in years that are not Geng years?
The month pillar of a Shen month takes its stem from the year stem’s pair, and only the Yi and Geng pair produces Jia Shen. Jia and Ji years open at Bing Yin and arrive at Ren Shen (壬申); Bing and Xin years arrive at Bing Shen (丙申); Ding and Ren years arrive at Wu Shen (戊申); Wu and Gui years arrive at Geng Shen (庚申). Each row comes straight from the deterministic lookup, checked on a real August date from one birth year in every family. Five year-stem pairs, five different Shen month pillars, and the pair your birth year belongs to decides which column applies.
Is it true that the Jia Shen month pillar appears only in Geng years?
No, and the mnemonic itself says so: the line 乙庚之岁戊寅头 pairs Yi with Geng, so Yi years deliver the identical Jia Shen month pillar. The lookup confirms it for 2025, a Yi Si year. The Geng year 2000 returns the identical pillar. Because the twelve branches cycle against the ten stems, the full sixty-pairing sequence of month pillars completes in five years, so a Jia Shen month returns every five years, alternating between Yi and Geng years. The recent run went 2015, 2020, and 2025, and the next Geng-year instance arrives in 2030. Reading the pillar as a Geng-years-only marker undercounts its appearances by half.
Which day stems meet helpful stars inside a Shen month, and how should you read them?
Table lookups name two day-stem families that meet a Shen month with helpful stars. For a Yi-stem Day Master, the Yi Gui Noble (天乙贵人) sits at the Rat and Monkey branches, and a Ji-stem Day Master reads the same pair. For a Bing-stem Day Master, the Wen Chang Scholar Star (文昌) sits at the Monkey (Shen), and Wu day stems read the same Monkey placement from the same classical table. Stars of this class color how a chapter tends to feel, they do not schedule events, and a careful reading weighs them alongside pillar strength and the season of the month branch. The Shen month, in other words, doubles as a helper-star station for more than one chart type.
Sources
Four Pillars of Destiny, Wikipedia: the four-pillar framework and the month pillar’s place in it.
Solar term, Wikipedia: the solar-term windows that bound every BaZi month.
Chinese Text Project: the open digital library holding the Ming-era treatises, including the San Ming Tong Hui, where the Five Tigers mnemonic is preserved.
Purple Mountain Observatory: the observatory whose calendar standards anchor this site’s solar-term data.
Conclusion
The lookup stays honest because the derivation stays mechanical: a Geng year opens at Wu Yin, seven counted steps land on Shen, and the Li Qiu to Bai Lu window decides whether a date qualifies. For your own birth chart, run the 12Zodiacs BaZi calculator and read the month pillar it returns against the anatomy above.
