TL;DR: Childbirth year checkers give different results because each site picks a different criterion within the same can chi system: heavenly stem, earthly branch by tam hợp, lục hợp or the clash and harm tables, or the nạp âm of the birth year, and uses a different year marker (1 January, Tết or Lập Xuân). The Tam Mệnh Thông Hội by Vạn Dân Anh discusses these can chi relations within a single Tử Bình chart and sets up no compatibility table between two people, so there is no common standard for the sites to follow.
Why does the same parents' birth year give a different result on each site?
The short answer: "tuổi" in the can chi system is not a single number but a bundle of information including the heavenly stem, the earthly branch and the nạp âm of the birth year, and each site picks one or several items from that bundle to compare. A site comparing heavenly stems sees compatibility, a site comparing earthly branches by tam hợp sees compatibility, and a site comparing nạp âm sees a clash. All three perform a real operation found in the books, but none is the "official method" of the whole Tử Bình discipline.
The Tam Mệnh Thông Hội (三命通會) by Vạn Dân Anh, of the Ming dynasty, was copied into the Tứ khố toàn thư. The Tứ khố summary identifies Vạn Dân Anh as the author, notes that for over two hundred years since the Ming those who discussed fate took this book as their compendium, and credits it with gathering the meanings of quan, ấn, tài, lộc, thực, thương and the good and bad omens of the thần sát[1]. The same summary faults the book for a one-sided view (偏執之見), because its arguments mostly take only chính quan, chính ấn and chính tài[1]. The book sets out many methods side by side rather than establishing a single compatibility table.
How are the heavenly stem and earthly branch of a birth year determined?
The heavenly stem of a birth year is one of the ten characters Giáp, Ất, Bính, Đinh, Mậu, Kỷ, Canh, Tân, Nhâm, Quý; the earthly branch is one of the twelve characters Tý, Sửu, Dần, Mão, Thìn, Tỵ, Ngọ, Mùi, Thân, Dậu, Tuất, Hợi. Pairing one stem with one branch along a 60-cycle produces sixty pairs from Giáp Tý, called the lục thập hoa giáp. The Tam Mệnh Thông Hội quotes Trần Đoàn on this pairing: a yang stem is matched with a yang branch and a yin stem with a yin branch, starting from Giáp Tý and proceeding in order to Quý Hợi (法以陽幹配陽枝陰幹配陰枝…自甲子為首…而盡於癸亥)[1].
The can chi year does not begin on 1 January, and there are two different markers. Tử Vi and the folk way of counting age follow the lunar year, which changes at Tết Nguyên đán, falling in late January or in February; Bát Tự changes the year pillar at the Lập Xuân solar term, on 3, 4 or 5 February in Vietnam time. This is the first source of discrepancy between sites: a person born in January or February may be given the previous year by one site and the next year by another, depending on whether the site uses 1 January, Tết or Lập Xuân.
Tam hợp, lục hợp, lục xung: three earthly branch comparisons giving three different results
This is the second layer and also the layer causing the most difference. Earthly branches do not stand alone but relate to one another in three main groups:
- Tam hợp: three branches combine into one cục, for example Thân, Tý, Thìn combine into the Thủy cục; Dần, Ngọ, Tuất into the Hỏa cục; Hợi, Mão, Mùi into the Mộc cục; Tỵ, Dậu, Sửu into the Kim cục.
- Lục hợp: branches pair up in harmony, such as Tý with Sửu, Dần with Hợi, Mão with Tuất, Thìn with Dậu, Tỵ with Thân, Ngọ with Mùi.
- Lục xung: branches pair up in clash, such as Tý clashing with Ngọ, Sửu with Mùi, Dần with Thân, Mão with Dậu, Thìn with Tuất, Tỵ with Hợi.
The Tam Mệnh Thông Hội discusses the five stem combinations (ngũ hợp thiên can) in its passage on combining stems, explaining them through yin-yang pairing: a yang stem acts as elder brother, a yin stem as younger sister, and the brother marries the sister to the phase that overcomes him, hence Ất combines with Canh, Đinh with Nhâm, Kỷ with Giáp, Tân with Bính, Quý with Mậu. This is the book's explanation, not a historical event. Ngũ hợp is a relation between heavenly stems, different from the lục hợp of earthly branches above.
The book also names each stem pair by virtue: Giáp combining with Kỷ is called "trung chính chi hợp", Ất combining with Canh is called "nhân nghĩa chi hợp", because Giáp is yang wood with the virtue of benevolence standing first among the ten stems, and Kỷ is yin earth, calm and sincere, with the virtue of nurturing things. This naming is the book's convention, not a scientific conclusion about human character.
Because several tables coexist, one pair of years can receive different conclusions depending on the table used. For example the Dần year and the Thân year: the two branches are not in the same tam hợp group (Dần belongs to the Dần, Ngọ, Tuất group; Thân to the Thân, Tý, Thìn group) and are not a lục hợp pair, but they are a lục xung pair. A site comparing only tam hợp and lục hợp will say neither compatible nor clashing, a site that also checks lục xung will say they clash. Some pairs are even contradicted by the tables: Dần and Hợi are a lục hợp pair, yet both sit in the tứ hành xung group of Dần, Thân, Tỵ, Hợi that many sites use instead of lục xung.
What is nạp âm and how do sites use it?
Nạp âm is the method of assigning one five-phase element to each can chi pair in the lục thập hoa giáp, for example Giáp Tý and Ất Sửu both belong to Hải Trung Kim; Bính Dần and Đinh Mão both belong to Lư Trung Hỏa. Childbirth year checkers often use nạp âm because it gives a single five-phase label for the whole birth year, easy to compare: metal overcomes wood, wood overcomes earth, earth overcomes water, water overcomes fire, fire overcomes metal.
Nạp âm has several ways of being established but one shared table. Thẩm Quát of the Northern Song, in Mộng Khê bút đàm volume 5, explained nạp âm through musical pitch: one pitch holds five notes, twelve pitches take in sixty notes, following the rule "the same kind takes a wife, every eight gives birth to a child" (同類娶妻，隔八生子)[2]. Hoàng Tông Hy of the early Qing, in Dịch học tượng số luận volume 1, section Nạp âm, wrote that although the nạp âm are the same, there are three ways of establishing the method (納音雖同，而立法有三)[2]. Scholars differ on the origin of the method, while the nạp âm name of each can chi pair is the same.
Thẩm Quát's explanation ties nạp âm to musical pitch, that is, to the assignment of an element to each can chi pair, and does not discuss compatibility between people. A site that uses nạp âm to compare the parents' years with the child's is applying that table to five-phase generating and overcoming, a step outside that explanation. The Tam Mệnh Thông Hội has a section, General discussion of nạp âm (總論納音), that copies the explanation of Mộng Khê bút đàm[1].
Where do the sites differ in their choices?
The differences can be grouped as follows:
| Choice | Practice | Consequence |
|---|---|---|
| Year marker | Use 1 January, Tết Nguyên đán or Lập Xuân | People born in January or February get different years |
| Comparison criterion | Only heavenly stem, only earthly branch, only nạp âm, or all three | The same pair of years comes out compatible, clashing, or neutral |
| Earthly branch method | Tam hợp, lục hợp, lục xung, tứ hành xung, lục hại: one table or several | The same branch pair gives different conclusions |
| Who is compared with whom | Only parents with child, or father with mother first then child | Some sites report a clash because the parents clash, others do not consider it |
| Weighting | Give more weight to heavenly stem, to earthly branch, or to nạp âm | Same data, different conclusion |
The table above is not from any ancient book; it is how Nhật Nguyệt classifies the choices seen in practice, and none of the choices in the table is forbidden by the Tam Mệnh Thông Hội.
Is any method in the book considered the standard for checking year compatibility?
No. The Tam Mệnh Thông Hội is a book of Tử Bình fate analysis; in volume 7, section Tử Bình thuyết biện, it writes "八字以日為主" (the eight characters take the day as the master), and then examines the day stem's relations with the other can chi in the four pillars, not a compatibility table between two people[1]. Checking husband-wife compatibility, or parent-child compatibility, is a layer of folk application, mainly based on tam hợp, lục hợp, the clash and harm tables and nạp âm. Being an application layer, it has no standard textbook, so each place chooses for itself.
The book discusses lục hợp and tam hợp in the sections on thần sát and on can chi relations, but discusses them as relations between can chi within one chart, not between one person's chart and another's[1]. This is where a clear distinction is needed: a method found in the book, and a method extended by sites beyond the book's scope, are two different things.
So how should one read those sites' results?
The most practical way is to see whether the site states its criterion clearly; if it only says in general that two years clash without indicating whether it relies on the heavenly stem, the earthly branch or nạp âm, and which year marker it uses, the reader cannot verify that result.
No research has proven that the parents' can chi years affect the health or character of the child born. The methods of tam hợp, lục hợp and nạp âm are conventions of an ancient knowledge system, with internal logic and learnable, but they are not medical or psychological conclusions. Therefore the sites' results should be read as cultural reference, not as medical indication.
Nhật Nguyệt's suggestion: when comparing results across sites, compare on the same criterion, because adding up conclusions from sites that use different criteria only produces a pile of contradictions. The Child's birth year by parents' ages tool shows the heavenly stem, earthly branch and nạp âm axes separately for the father's and the mother's birth years. The actual date and time of birth is decided by the health of mother and child, following the doctor's advice.
