TL;DR: Lục Nhâm derives the Three Transmissions (Initial, Middle, Final) from the Four Lessons on the Heaven and Earth plates through nine methods examined in order: tặc khắc, tỷ dụng, thiệp hại, dao khắc, mão tinh, biệt trách, bát chuyên, phục ngâm, phản ngâm. The first three apply when a lesson has its upper and lower characters overcoming each other; the next four apply when no lesson does; the last two have their own procedures when the Heaven plate coincides with or stands opposite the Earth plate. The texts agree on the first five; for the last four they differ: the Đại Lục Nhâm linh giác kinh puts bát chuyên before biệt trách, the Đại Lục Nhâm kim giảo tiễn the other way round.
What Are the Nine Methods for Deriving the Three Transmissions in Lục Nhâm?
In Lục Nhâm, after setting up the Heaven and Earth plates (Thiên bàn, Địa bàn) and the Four Lessons (Tứ khóa), the practitioner must select the three transmissions, Initial, Middle and Final (Sơ, Trung, Mạt), in order to read the lesson. The nine methods for deriving the Three Transmissions are nine rules for finding the Initial Transmission (also called phát dụng), examined one by one in a fixed order: tặc khắc, tỷ dụng, thiệp hại, dao khắc, mão tinh, biệt trách, bát chuyên, phục ngâm, phản ngâm. Volume one of the Đại Lục Nhâm linh giác kinh (大六壬靈覺經) states: "古经九宗门乃一贼克，二比用，三涉害，四遥克，五昴星，六八专，七别责，八伏吟，九返吟", that is, the nine gates of the old classic, in which bát chuyên comes before biệt trách.
The nine methods fall into three groups according to the shape of the Four Lessons. Tặc khắc, tỷ dụng and thiệp hại apply when a lesson has its upper and lower characters overcoming each other. Dao khắc, mão tinh, biệt trách and bát chuyên apply when no lesson does. Phục ngâm and phản ngâm have their own procedures when the Heaven plate coincides with, or stands opposite to, the Earth plate in every palace. The Linh giác kinh moves the last two elsewhere, so its first volume "只有七体" (has only seven types); this is why some texts speak of seven methods and others list all nine.
Why Is There an Order of Priority Among the Methods?
The purpose of the order of priority is to ensure that each lesson yields one single set of Three Transmissions, independent of the practitioner. Without an order, two Lục Nhâm masters could choose two different sets of Three Transmissions for the same lesson, and the entire system of divination would lose its reproducibility.
The general principle is to examine from top to bottom; when a method meets its condition, stop and do not examine the methods below. Lục Nhâm texts usually write the method name at the head of each lesson: the Lục Nhâm trực chỉ (六壬直指) records the lesson of a Đinh Sửu day, fourth configuration, with Thìn above the stem as "课体：昴星，掩目，虎视", meaning a lesson derived by mão tinh, with the secondary names yểm mục and hổ thị.
Conditions and how the Initial Transmission is taken for each method:
| Method | When it applies | How the Initial Transmission is taken |
|---|---|---|
| Tặc khắc | Exactly one lesson with the lower character overcoming the upper; failing that, exactly one with the upper overcoming the lower | The upper character of that lesson |
| Tỷ dụng | Two, three or four lessons contain an overcoming | The upper character sharing the day stem's yin or yang |
| Thiệp hại | The overcoming lessons all share, or all fail to share, the day stem's yin or yang | The upper character of the lesson overcome more often on the way back to its home palace |
| Dao khắc | No lesson has its upper and lower characters overcoming each other | An upper character overcoming the day stem; failing that, one overcome by the day stem |
| Mão tinh | All four lessons present, no overcoming, no dao khắc | Taken from the Dậu palace: look up on a yang day, look down on a yin day |
| Biệt trách | Only three distinct lessons, no overcoming, no dao khắc | Yang day by the stem combining with the day stem, yin day by the three-harmony group of the day branch |
| Bát chuyên | Day stem and branch share a position, only two distinct lessons, no overcoming | Yang day count forward from lesson one, yin day count backward from lesson four |
| Phục ngâm | Heaven plate coincides with Earth plate | By overcoming if any; otherwise the character above the stem (yang day) or above the branch (yin day) |
| Phản ngâm | Heaven plate opposite Earth plate | By overcoming if any; Đinh, Kỷ, Tân days on Sửu or Mùi without overcoming take Hợi or Tị |
How Do Tặc Khắc and Tỷ Dụng Differ?
Tặc khắc is the first method examined. According to the definition in Lục Nhâm, volume 1: Nhập môn, a tặc lesson is a lesson in which the lower character overcomes the upper character; for instance when the lower character is Dậu and the upper character is Mão, the lesson is a tặc lesson, because Dậu metal below overcomes Mão wood above[1]. Conversely, a khắc lesson is a lesson in which the upper character overcomes the lower character, for example Tý above overcoming Ngọ below[1].
If exactly one lesson is a tặc lesson, its upper character becomes the Initial Transmission, and the lesson is called trùng thẩm (重審); if there is no tặc lesson and exactly one khắc lesson, its upper character is taken, and the lesson is called nguyên thủ (元首). The Đại Lục Nhâm kim giảo tiễn (大六壬金鉸剪) sums this up in the verse "取课先从下克呼，若无下克上为初": look first for a lower character overcoming the upper, and only if there is none take the upper overcoming the lower. The Middle Transmission is the Heaven-plate character standing over the Earth-plate position of the Initial Transmission, and the Final Transmission is the Heaven-plate character standing over the position of the Middle.
Tỷ dụng applies when two, three or four lessons all contain an overcoming. The Kim giảo tiễn verse reads "常将天日相比取，阳日用阳阴用阴": among those lessons (the tặc lessons first, if there are any), take the upper character that shares the yin or yang of the day stem, a yang character on a yang day and a yin character on a yin day. A lesson derived this way is also called tri nhất (知一).
When Are Thiệp Hại and Dao Khắc Used?
Thiệp hại applies when the overcoming lessons all share the day stem's yin or yang, or all fail to, so tỷ dụng cannot decide. The Linh giác kinh treats "涉归本家" as the proper method: from where the upper character stands, travel across the Earth plate back to its home palace and count the overcomings met along the way; the lesson that suffers more, that is, is harmed more deeply, is taken. The shortcut is to see whether the upper character stands on a mạnh palace (Dần, Thân, Tị, Hợi), a trọng palace (Tý, Ngọ, Mão, Dậu) or a quý palace (Thìn, Tuất, Sửu, Mùi) of the Earth plate, following "孟深仲浅季又浅": mạnh is the deepest. Taking the character on a mạnh palace is called kiến cơ (見機), on a trọng or quý palace sát vi (察微). If the lessons are equally deep, the Chu dịch hàm thư ước tồn (周易函書約存) takes the character above the stem on a yang day and the character above the branch on a yin day.
Dao khắc applies when no lesson has its upper and lower characters overcoming each other. "Dao" (遙) means distant: instead of overcoming within one lesson, the upper characters of the lessons are compared with the day stem. The Chu dịch hàm thư ước tồn takes an upper character that overcomes the day stem as the Initial Transmission, called hao thỉ (蒿矢); failing that, an upper character overcome by the day stem, called đạn xạ (弹射); if two characters overcome the stem, or the stem overcomes two, the one sharing the day stem's yin or yang is chosen.
What Is Special About Mão Tinh and Biệt Trách?
Mão tinh (昴星) applies when all four lessons are present but there is neither overcoming nor dao khắc. It uses the Dậu palace as its marker; the Kim giảo tiễn notes that the Mão mansion lies in Dậu. On a yang day, look up from Dậu on the Earth plate and take the Heaven-plate character above it as the Initial Transmission, with the character above the branch as Middle and the character above the stem as Final. On a yin day, look down from Dậu on the Heaven plate and take the Earth-plate character beneath it, with the character above the stem as Middle and the character above the branch as Final.
The Lục Nhâm trực chỉ records the lesson of a Đinh Sửu day, fourth configuration, Thìn above the stem, with the judgement "此昴星之卦，名冬蛇掩目，亦名虎视": a mão tinh lesson called "the winter snake covering its eyes", also called hổ thị. The judgement advises the enquirer to behave like a hibernating insect, withdraw into hiding and guard with utmost care; for a night reading, snake and tiger both run rampant, an even more inauspicious image.
Biệt trách (別責) applies when the Four Lessons contain only three distinct lessons (two of them coincide), with neither overcoming nor dao khắc. The Kim giảo tiễn says: on a yang day take the Heaven-plate character above the home palace of the stem that combines with the day stem; on a yin day take the character that follows the day branch, in forward order, within its three-harmony group (a Dậu day takes Sửu from Tị, Dậu, Sửu); the Middle and Final Transmissions both take the character above the stem. The Chu dịch hàm thư ước tồn counts nine biệt trách lessons in all: "别责有九课".
How Do Bát Chuyên, Phục Ngâm and Phản Ngâm Differ?
Bát chuyên (八專) applies to days whose stem and branch share a position, that is, the home palace of the day stem is the day branch itself (such as Giáp Dần, Canh Thân, Đinh Mùi, Kỷ Mùi), so the Four Lessons reduce to two distinct lessons. If those two lessons contain an overcoming, tặc khắc, tỷ dụng and thiệp hại still apply; if not, the Kim giảo tiễn verse "两课无克号八专，阳日阳神顺行三，阴日阴神逆三数" applies: on a yang day count forward from the upper character of lesson one to the third position, on a yin day count backward from the upper character of lesson four to the third position, and take that as the Initial Transmission; the Middle and Final Transmissions both take the character above the stem.
Phục ngâm (伏吟) is a lesson in which the Heaven plate coincides with the Earth plate palace by palace, every character staying in place. According to the Chu dịch hàm thư ước tồn, if there is an overcoming, it gives the Initial Transmission; if not, take the character above the stem on a yang day and the character above the branch on a yin day, then the Middle is the character the Initial punishes (hình) and the Final the character the Middle punishes, with a separate rule for self-punishment. The Lục Nhâm trực chỉ records the lesson of a Giáp Tý day, first configuration, Dần above the stem, as "课体：伏吟，元胎", and its judgement calls it "自任之卦" (a lesson of self-reliance), "利静不利动" (favourable for stillness, not for movement).
Phản ngâm (返吟) is a lesson in which every Heaven-plate character stands on the palace opposite it on the Earth plate. Phản ngâm usually contains overcomings, so the Initial Transmission is taken as in tặc khắc, tỷ dụng and thiệp hại; the difference is that the Middle is the character opposite the Initial and the Final the character opposite the Middle. On Đinh, Kỷ and Tân days falling on Sửu or Mùi, a phản ngâm without overcoming is handled separately: a Sửu day takes Hợi and a Mùi day takes Tị as the Initial Transmission (Chu dịch hàm thư ước tồn). The Linh giác kinh calls these lessons tỉnh lan xạ (井栏射) and names five days: Đinh Sửu, Đinh Mùi, Kỷ Sửu, Tân Sửu, Tân Mùi.
Is the Order of the Methods Consistent Among Texts?
Not entirely. For the first five methods (tặc khắc, tỷ dụng, thiệp hại, dao khắc, mão tinh), the Linh giác kinh, the Kim giảo tiễn and the Chu dịch hàm thư ước tồn agree. The remaining four differ: the Linh giác kinh, following the old classic, puts bát chuyên sixth and biệt trách seventh; the Đại Lục Nhâm kim giảo tiễn puts biệt trách sixth and bát chuyên seventh; the Chu dịch hàm thư ước tồn moves phục ngâm and phản ngâm up to sixth and seventh and leaves biệt trách and bát chuyên at the end.
Each of these four methods depends on a particular shape of the lesson (fewer than four distinct lessons, or a Heaven plate coinciding with or opposite the Earth plate), so when comparing examples across texts the student needs to know how each text orders them.
Nhật Nguyệt's Suggestion
When studying the nine methods for deriving the Three Transmissions, first master how the Four Lessons are set up, because every method begins by checking whether each lesson contains an overcoming and whether there are four distinct lessons. Practise tặc khắc and tỷ dụng on simple lessons before moving to the methods for lessons without overcoming. With phục ngâm and phản ngâm, remember that the Initial Transmission may still be taken by overcoming; what sets these two apart lies mainly in the Middle and Final Transmissions.
Remember that this is a convention of the Lục Nhâm discipline, not a natural law. The classical texts do not agree entirely on the order and details of each method, so when comparing with others, state clearly which text you follow.
