TL;DR: Thấu can (透干) means a stem appears among the four pillar stems; thông căn (通根) means that stem has a root in an earthly branch below. A stem that is exposed but rootless is weak and easily controlled, while a stem that is rooted but not exposed holds latent force until a luck period brings it out. Tử Bình chân thuyên and Trích Thiên Tủy both use these two criteria as the first measure when judging strength and choosing the dụng thần.
What Are Thấu Can and Thông Căn?
Thấu can (透干) is when a heavenly stem appears clearly among the four stems of the four pillars (year stem, month stem, day stem, hour stem). Thông căn (通根) is when that stem has a root in an earthly branch, meaning the branch contains a stem of the same element. In short: thấu can is exposure on the outside, thông căn is rooting on the inside. The two conditions are independent; a stem can be exposed without being rooted, rooted without being exposed, or possess both.
Tử Bình chân thuyên, in its discussion of the ten stems, opens with: "Địa chi sở tàng chi can, bản tĩnh dĩ đãi dụng, thấu xuất can đầu, tắc hiển kỳ dụng hĩ. Cố can dĩ thông căn vi mỹ, chi dĩ thấu xuất vi quý" (地支所藏之干，本靜以待用，透出干頭，則顯其用矣。故干以通根為美，支以透出為貴)[1]. This means stems hidden in earthly branches lie still awaiting use, and only when they emerge among the stems does their function become clear; therefore stems take thông căn as their beauty, and branches take exposure as their value. This is the original definition, and also why the two concepts always travel as a pair.
Which Stems Count as Thấu?
The four pillar stems are four positions: year stem, month stem, day stem, hour stem. Any stem in those four positions is thấu. The day stem (nhật chủ) is always thấu because it is the subject of the chart; the other three stems are the thấu of their corresponding thập thần. In practice, people care most about two kinds of thấu: the thấu of the dụng thần and the thấu of stems that can break the cách cục.
One easily overlooked point: exposure alone does not mean strength. A stem exposed at the month stem (nguyệt can) usually carries more weight than the year stem, because the month is the season's command and is structurally closer to the nhật chủ. The hour stem and year stem are farther away, their force more dispersed. Thiên Lý mệnh cảo by Vi Thiên Lý (1935), when classifying cách cục by nguyệt lệnh, repeatedly uses the pattern "if stem X is exposed, it forms cách X; if not exposed, one may still choose among the hidden stems". This shows that thấu is a priority condition for fixing the cách, but not the only condition.
What Types of Thông Căn Are There?
Từ Lạc Ngô's commentary in the 1936 annotated edition distinguishes four types of root: lộc (the stem meets a branch that is its own lộc, for example Giáp meeting Dần), vượng (đế vượng, for example Giáp meeting Mão), trường sinh (for example Giáp meeting Hợi), and mộ khố (for example Giáp meeting Mùi, since Mùi is the tomb of Wood). There is also dư khí, the leftover qi of the previous element in a branch, for example Thìn containing Ất wood, the dư khí of spring[1].
The table below summarizes the types of root by element, cross-referenced with the table of nhân nguyên tư lệnh (人元司令) in the same source:
| Type of root | Definition | Example with Wood | Note |
|---|---|---|---|
| Lộc | Stem meets its lộc branch | Giáp meets Dần, Ất meets Mão | Strongest root, stem qi is pure |
| Đế vượng | Stem meets a branch at đế vượng | Giáp meets Mão, Ất meets Dần | Strength comparable to lộc |
| Trường sinh | Stem meets a branch at trường sinh | Giáp meets Hợi | Newly born qi, has force but not yet full |
| Mộ khố | Stem meets the tomb branch of its element | Giáp meets Mùi | Qi stored away, needs clash or combination to emerge |
| Dư khí | Branch retains qi of the previous element | Ất hidden in Thìn | Weaker than the three above |
Why Is Thông Căn More Important Than Thấu Can?
In the chart, earthly branches are the foundation, heavenly stems the tip. A stem without a root is like a tree planted on rock: it looks fresh but cannot draw water. A rooted stem, even if not exposed, is still real force, waiting only for a luck period to emerge. Từ Lạc Ngô writes in his commentary: "Thiên can thông căn, bất cận lộc vượng vi mỹ, trường sinh, dư khí, mộ khố giai kỳ căn dã" (天干通根，不僅祿旺為美，長生、餘氣、墓庫皆其根也)[1]. He also criticizes the notion that "meeting a tomb always requires a clash" as an error of shallow books, because Thìn is originally the Wood land of the East, and in the twelve days after Thanh Minh, Ất wood still holds command, its dư khí still strong, so it cannot be called entering the storehouse[1].
In the same note, Từ Lạc Ngô quotes the Trích Thiên Tủy: "Thiên toàn nhất khí, bất khả sử địa đức mạc chi tải; địa toàn tam vật, bất khả sử thiên đạo mạc chi dung" (天全一氣，不可使地德莫之載；地全三物，不可使天道莫之容)[1]. This means if the four stems are all of one element, the earthly branches must be able to carry it; if the branches contain all three stems, the heavenly stems must be able to contain them. Without that correspondence, the configuration becomes lopsided and hard to use.
What If a Stem Is Exposed but Not Rooted?
A stem that is exposed but not rooted is called "hư thấu" or "phù phiếm". Từ Lạc Ngô's note gives the example of four pillars all being Tân Mão or all being Bính Thân: although stem and branch share the same qi, the stem has no root in the branch (Tân meeting Mão is tuyệt địa, Bính meeting Thân is also bệnh địa), so it is not noble enough[1]. In practice, a hư thấu stem can still be used if it is supported by another stem or branch, but by itself it is not strong enough to carry the role of dụng thần.
What If a Stem Is Rooted but Not Exposed?
A stem hidden in a branch that is not exposed is called "tàng can" or "phục thần". It is still real force, but that force lies dormant. The book says "địa toàn tam vật, bất khả sử thiên đạo mạc chi dung": if a branch contains three stems and none is exposed, its function is not manifest. In luck cycles, when the đại vận or lưu niên brings that stem up to the stem level, it will act. This is the basis of the "waiting for luck" technique in Tử Bình: a dụng thần that is hidden and not exposed often corresponds to an early period of life before it emerges, and only brightens when the luck period exposes it.
Thấu Can and Thông Căn in Fixing the Cách Cục and Choosing the Dụng Thần
Vi Thiên Lý in Thiên Lý mệnh cảo (1935) presents a table classifying cách cục by nguyệt lệnh, in which the first criterion is which stem is exposed. For example: "Giáp born in the month of Thân, if the stem Canh metal is exposed it is the thất sát cách, if Mậu earth is exposed it is the thiên tài cách, if Nhâm water is exposed it is the thiên ấn cách; if none of Canh, Nhâm, Mậu is exposed, one may still choose among the hidden stems". Likewise, "Bính born in the month of Dần, if the stem Giáp wood is exposed it is the thiên ấn cách, if Mậu earth is exposed it is the thực thần cách". This table shows that thấu is the number one priority condition for fixing the cách, and only when no stem is exposed does one consider the hidden stems.
Nhậm Thiết Tiều in Trích Thiên Tủy xiển vi, in the "Chân thần" section, defines: "Chân giả, đắc thời bỉnh lệnh chi thần dã; giả giả, thất thời thoái khí chi thần dã. Ngôn nhật chủ sở dụng chi thần, tại đề cương tư lệnh, hựu thấu xuất thiên can, vị tụ đắc chân" (真者，得時秉令之神也；假者，失時退氣之神也。言日主所用之神，在提綱司令，又透出天干，謂聚得真). This means the dụng thần that both holds command in the month branch and is exposed in the heavenly stems is called "tụ đắc chân", that is, possessing both thông căn and thấu can. This is the most ideal case. Conversely, if the dụng thần only holds command but is not exposed, or is exposed but does not hold command, its force is weaker and easily broken.
A Specific Example from the Books
Nhậm Thiết Tiều cites a chart: Giáp Tý, Bính Dần, Kỷ Sửu, Giáp Tý (山东刘中堂造). He analyzes: Kỷ earth born in early spring, the qi is cold and damp, the self is weak; it gets Giáp and Bính both exposed, the chính ấn and chính quan are clear, "tụ đắc chân". There is no metal in the pillars so it is not transformed, the giả thần does not cause disorder; and it meets the southeast luck period where the ấn is strong, so he rose to Thượng thư. Here, Giáp wood (chính quan) and Bính fire (chính ấn) are both exposed at the stem level, and Giáp has a root in Dần (lộc), Bính has a root in Dần (trường sinh). That is the model of thấu can thông căn.
Another chart: Nhâm Thân, Nhâm Dần, Bính Tý, Ất Mùi (铁制军造). Nhậm Thiết Tiều comments: the sát is strong, the tài is strong, the young wood receives support, and what is most favorable is that Dần wood, the chân thần, holds command, and the hour stem exposes Ất wood, the nguyên thần. Dần and Thân clash, which is called "hữu bệnh". When the luck period reaches the southern fire land, it removes the ailment of Thân metal, so he rose to phong cương. Here, Ất wood is exposed at the hour stem and rooted in the Dần month branch, which holds command (Dần stores Giáp wood), and in Mùi (Mùi stores Ất). Thấu and thông căn are both present, but because of the Dần Thân clash it must wait for a luck period to resolve.
Where Do the Schools Disagree?
One long-debated point is whether the mộ khố needs a clash to emerge. Tử Bình chân thuyên criticizes the notion "meeting a storehouse always requires a clash" as "mậu ngộ khả xuy" (謬誤可嗤), arguing that Thìn is the Wood land of the East, and in the twelve days after Thanh Minh, Ất wood still holds command, so it cannot be called entering the storehouse; and Thìn Tuất Sửu Mùi are for earth the original qi, with no storehouse to speak of[1]. This view is not shared by all Tử Bình masters. Some schools still keep the rule "mộ khố phùng xung nhi phát", and in practice many charts show that the mộ khố only acts when it meets a clash or combination.
Another point is the role of yin stems and yang stems in thông căn. Từ Lạc Ngô writes: "Dương trường sinh hữu lực, nhi âm trường sinh bất thậm hữu lực, nhiên diệc bất nhược. Nhược thị phùng khố, tắc dương vi hữu căn, nhi âm vi vô dụng" (陽長生有力，而陰長生不甚有力，然亦不弱。若是逢庫，則陽為有根，而陰為無用)[1]. He also criticizes the notion that yin stems have their own trường sinh and that the mộ khố is useless as a misunderstanding of yin-yang trường sinh, leading to inconsistency. This view is also not accepted by all schools.
Do the Day Stem and Other Stems Differ?
Tử Bình chân thuyên states clearly: "Hựu thử tiết tuy chỉ nhật chủ, nhi niên nguyệt thời chi can giai đồng, năng đắc nguyệt lệnh chi khí, tự vi tối cường; phủ tắc, nguyệt lệnh hưu tù, nhi niên nhật thời chi trung, đắc sinh lộc vượng dư khí mộ, giai vi thông căn dã" (又此節雖指日主，而年月時之干皆同，能得月令之氣，自為最強；否則，月令休囚，而年日時支中，得生祿旺餘氣墓，皆為通根也)[1]. This means the thông căn principle applies to all stems, not only the nhật chủ. A stem that obtains the qi of the nguyệt lệnh is strongest; if the nguyệt lệnh is hưu tù, then roots in the year, day, and hour branches still count as thông căn.
Nhật Nguyệt's Suggestions When Reading a Chart
Nhật Nguyệt's suggestion: when reading a chart, follow this order. First, identify the nhật chủ and its element, then find whether the nhật chủ has a root in any branch, and whether that root is lộc, vượng, trường sinh, mộ khố, or dư khí. Second, examine the remaining stems in the four pillars, which are exposed and which are rooted. Third, cross-check with the nguyệt lệnh to see which stem holds command. Fourth, only then discuss the cách cục and dụng thần.
A hư thấu stem that holds command can still be used, but needs support. A rooted stem that is not exposed usually corresponds to unemerged potential, waiting for luck. A stem that is both exposed and rooted is real force, but if it is clashed or lost through combination, that force also declines. These are guiding principles, not fixed formulas, and each specific chart must be considered as a whole.
In Summary
Thấu can and thông căn are the two axes for measuring a stem's force in the four pillars. Thấu can tells whether the stem is manifest; thông căn tells whether the stem has a foundation. Tử Bình chân thuyên takes "can dĩ thông căn vi mỹ, chi dĩ thấu xuất vi quý" as its foundational principle; Trích Thiên Tủy xiển vi takes "tụ đắc chân" as the standard for the dụng thần. The schools still disagree on the mộ khố and on yin-yang trường sinh, but all agree that a stem without a root finds it hard to carry a key role in the chart.
