Thirty Days Behind One Branch
Four characters shape a BaZi chart, and three of them settle on one meaning each. The month branch does not. That month’s days are shared out among several stems, and which one covers the day you were born is a choice the chart never shows. Two people born in the same month, under the same branch, can be handed different stars.
The Day Nobody Wrote Down
The eight characters come from year, month, day and hour. Naming the star inside the month branch takes one more thing: how far into the month you arrived, reckoned from the day it began. The branch is in your chart. The day inside it is not. Every reading that names a star for your month has already spent that reckoning silently.
That a branch holds several stems the tradition states plainly; I take it as given and look only at what varies, the days.
The Marginal Note Spells It Out
An annotated edition of 《渊海子平》 gives the procedure without hedging, in a marginal note attached to the passage on the months: 子宫初一日生至初十日生，当取壬水，十一日生至三十日生方取癸水，余仿此。 Born between the first and tenth day of a 子 month, take 壬水; from the eleventh to the thirtieth, take 癸水; and so on for the rest.
For a 甲 day master, yang wood, 壬 is water feeding wood with signs matching, which later charts call 偏印, Indirect Resource; 癸, opposite in sign, they call 正印, Direct Resource, though the book’s own gloss of the ten names it 印绶. Ten days of one star and twenty of another, inside a single branch.
One Book, Two Allotments
The same book holds two passages for the year, and they do not agree. 《论天干地支暗藏总诀》 opens:
立春念三丙火用，余日甲木旺提纲。
From 立春, 念三 days of 丙火, the rest given to 甲木; 念 stands for twenty, so twenty-three. A few lines below, 《又论节气歌》 hands the same month out differently:
寅宫戊丙各七朝，十六甲木方堪器。
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戊 seven days, 丙 seven, 甲 sixteen. So 丙火 holds twenty-three days in the first passage and seven in the second. Both appear in the same book; the marginal note says only that the first reckons from 立春 itself.
Another Book Keeps Different Stems
《三命通会·论人元司事》 arranges the year a third way: 正月建寅，寅中有艮土用事五日，丙火长生五日，甲木二十日。 Five days, five and twenty. It calls the earth of 寅 by the name 艮土, the earth of a trigram, never saying which earth stem is meant.
The larger difference sits in 未, 戌 and 丑. Where 《又论节气歌》 puts a yin stem there, this chapter puts the yang one, and names it as the stem whose tomb that branch is.
- 未: 乙 for three days, against 甲 for five
- 戌: 丁 for three days, against 丙 for five
- 丑: 辛 for three days, against 庚 for five
Both books give each of those months thirty days. The days agree. The stems do not.
The Star Changes Midway
Now set a day master against it. Take 甲, yang wood, born in a 戌 month. Under 《又论节气歌》 the middle stem is 丁, yin fire: wood feeds fire, signs differ, so 伤官, Hurting Officer. Under 《三命通会》 it is 丙, yang fire: signs match, so 食神, Eating God. Same month, same branch, two stars the tradition treats very differently.
The same substitution in 丑 moves 正官 to 七杀, in 未 moves 劫财 to 比肩. A yin stem and a yang stem occupy the same place, and the star flips with it.
Under the first passage above, a 寅 month carries no earth at all for twenty-three of its thirty days, so a 甲 day master finds no wealth star in it.
Living With Three
Nothing here settles which arrangement is right; the books do not, and I will not. What is settled is narrower and still useful: the star your month contributes cannot be taken from the eight characters alone. It needs a day-by-day allotment and a set of stems, and both vary between sources treated as classical.
So when a reading names a star for your month, ask which of them produced it. Most tools will not say. The Ten Gods guide at AskingMing sets out the ten names and the two groups they fall into, those that share your yin-yang sign and those that take the other one, enough to check which star any given stem becomes for you, though it does not take up the days of the month.
The calculator at askingming.com turns year, month, day and hour into the four pillars and reads the Ten Gods alongside them, so you can see which star it assigned to your month and ask about it.
