What are hidden stems (藏干)?

Why a branch would hide anything

The ten Heavenly Stems are single, clean elemental forces — one stem, one element, one polarity. The twelve Earthly Branches are not. A branch is a span of time: a month, a two-hour period, a segment of a year. Time is mixed. The month we call 寅 is early spring, and early spring is mostly emerging Wood, with the last warmth of the Fire that is coming and a residue of the Earth that was.

Hidden stems (藏干, cáng gān) are how that mixture is written down. Each branch conceals one to three Heavenly Stems, and those stems carry the same elemental forces as visible ones — at reduced weight, and out of sight.

Across the twelve branches there are twenty-eight hidden stems in total. Three branches — 子, 卯, and 酉 — are pure and hide a single stem each. Seven hide three: the four Earth branches 辰戌丑未, called the storehouses because they hold the residue of a whole season, plus 寅, 巳, and 申. The remaining two, 午 and 亥, hide two.

Main, middle, and residual qi

Where a branch hides more than one stem, they are not equal. The first is the 本气 (běn qì), the main qi — the branch's own dominant nature, and the one that does most of the work. The second is the 中气 (zhōng qì), middle qi, usually the element the branch is storing or transitioning toward. The third is the 余气 (yú qì), residual qi, the trace of what preceded.

So 寅 hides 甲 (main, Wood), 丙 (middle, Fire — spring warming), and 戊 (residual, Earth). Reading it as "寅 contains Wood, Fire and Earth equally" loses the entire point: the branch is Wood that is turning toward Fire, with Earth left over.

One convention worth naming, because sources vary. Some traditions weight hidden stems by 人元司令 — how deep into the solar term the birth falls, which decides which hidden stem is "on duty" at that moment. That refinement is outside our stated scope. We read the three-tier structure and say so, rather than applying a finer division inconsistently.

Where this table comes from, and where the sources split

The table above is the one every modern calculator prints, and it traces back to a single verse in the Yuanhai Ziping (淵海子平): 「子宫癸水在其中，丑癸辛金己土同」, "in 子 the Water of 癸; in 丑, 癸, the Metal of 辛 and the Earth of 己". The open-source engine we chart with reproduces that verse stem for stem, main qi first. On what a branch contains, the verse and the engine agree on all twelve branches.

They stop agreeing on the other question: which hidden stem is in command on which days of the month (人元司令). The Sanming Tonghui (三命通会), the Ming manual collected in the imperial Siku Quanshu, gives two tables in the same section, and they differ from each other. Its main text puts a yang stem in the four storehouse branches (壬 in 辰, 庚 in 丑, 甲 in 未, 丙 in 戌) where every other table has the yin one. Neither of its tables has 己 inside 午. The days given to 甲 in 寅 are 20, 18 or 16, depending on the table.

The same section records a critic who rejected the day counts outright: 「又豈可以三五七日為限哉」, how can it be limited to three, five or seven days? The disagreement is old, and it is why we read the three-tier structure and leave the day-by-day command out. The full comparison, seven sources branch by branch with the quotes, is published as open data; the link is at the end of this page.

What hidden stems are actually used for

Two things, and both matter more than the table itself.

First, rooting (通根). A Heavenly Stem standing alone in the chart is exposed — a force with no ground under it. If that same element appears as a hidden stem in one of the branches, the visible stem is said to be rooted there, and rooting is much of what separates a strong Day Master from a weak one. This is why a chart can look Wood-heavy on its four visible stems and still read as weak: nothing underneath.

Second, the Ten Gods. Hidden stems generate relationships to the Day Master exactly as visible stems do, which is where the "secondary stars" (副星) in a chart come from. A branch that looks inert on the surface may be carrying a Wealth or Officer relationship inside it, and much of the texture of a reading lives in those.

Do hidden stems count as much as visible ones?

No, and no serious method treats them as equal. A visible stem acts openly; a hidden stem acts through the branch that contains it, at a discount that increases from main to middle to residual qi.

How much of a discount is where methods differ, and the differences are not small. Our own weighting — including how the month branch is treated, since it carries more force than the other three — is written out on the charting-conventions page. If you see a strength verdict without the weighting behind it, the verdict is not reproducible, which means it is not really a measurement.