What is 용신 (yongsin)?
The one element this chart needs most to sit level.
In plain words
Think of your chart as a room with five things in it — wood, fire, earth, metal, water — piled up in uneven heaps. You are one of those things: the element of your day stem. Sometimes the room is stacked in your favour and you have more push than you can spend. Sometimes it is stacked against you and you are carrying more than you have strength for. Yongsin is the answer to one question: which of the five would even the room out?
The classical rule is one sentence: restrain what is strong, support what is weak. If you were born with a lot of push, the element that drains or checks you is the helpful one. If you were born with little, the element that feeds you is. That is why a yongsin is not a lucky charm — it is whatever this particular room is short of.
How Bokki computes it
First we weigh the room. Each upper letter counts 1. Each lower letter is split across the stems hidden inside it, so a Water day stem sitting on a branch that hides 壬 or 癸 really is rooted, not just apparently so. The month's lower letter gets three extra multiples because the season decides most of it. A full trine of three lower letters adds 3 to that element; a half trine holding the cardinal branch adds 1.5; a full seasonal trio adds 3.
Then we split those weights in two. On one side sits what stands with you and what feeds you (비겁 + 인성); on the other, what you make, what you handle, and what presses you (식상 + 재성 + 관성). If the first side holds more than 55% of the total, the chart reads strong (신강). Below 45%, it reads weak (신약). In between, balanced (중화). A weak chart takes the element that generates it, then itself. A strong chart takes whichever of drain, wealth and pressure the chart is shortest of. A balanced chart takes the two scarcest of all five.
There is a second layer on top: temperature (조후). A chart born in the summer months runs hot and asks for Water; a chart born in the winter months runs cold and asks for Fire. When that need is real, it moves to the front of the list. We keep the pre-temperature answer separately, so the reading can honestly say “by strength it would be Earth, but this chart runs hot, so Water comes first.” The first element on the final list is the 용신; the second is what tradition calls the 희신.
Three charts run through the actual code. Born 15 May 1990 14:30 Seoul: 庚午 辛巳 庚辰 癸未, a 庚 day stem, support share 0.436 — weak. By strength alone the answer would be Earth then Metal, but a 巳 month runs hot, so the final list is Water, Earth, Metal, and the element to go easy on is Fire. Born 2 November 1988 09:10 Busan: 戊辰 壬戌 辛酉 癸巳, a 辛 day stem at 0.728 — strong, temperate season, so the list is Wood, Fire, Water and the element to go easy on is Earth. Born 3 February 2001 23:50 Seoul: 庚辰 己丑 丁酉 壬子, a 丁 day stem at 0.093 — very weak, and a 丑 month is cold, so Fire leads and Wood follows.
What we do not compute
We use two layers only — 억부 (restrain and support) and 조후 (temperature). Tradition has other ways of choosing a yongsin: 병약, 통관, and the special charts where a very weak day stem is read as surrendering to the dominant element (종격) rather than needing support. Bokki does not test for any of those. A chart that a 종격 reader would treat one way, we read by strength like any other.
Schools disagree about yongsin, and they have disagreed for centuries. Two experienced readers can look at the same eight letters and name different elements, because they weigh the month, the rooting and the temperature differently. Our answer is one defensible reading with its arithmetic written down — not the only correct one. If a reader you trust says something else, they are not necessarily wrong.
And the 용신 is not a verdict about you. We use it to say which days lean with you and which ask you to be steadier — nothing more. We do not put a number on how your life will go, and we do not tell you what to do about other people.
Closing note — one strength test, two layers on top
To sum up: yongsin comes from one strength test — support (비겁+인성) as a share of the whole chart, split at 55% and 45% — with temperature laid over it afterward. The three worked charts above land at support shares of 0.436, 0.728 and 0.093 — weak, strong and very weak — and in every one of them the 억부 answer and the 조후 answer are kept as two separate lists, so the page can say plainly when temperature overrides strength, as it does for a hot 巳 month or a cold 丑 month.
We only test two layers — 억부 and 조후 — and say nothing about 병약, 통관, or the special 종격 charts that read a very weak day stem as surrendering rather than needing support. Two experienced readers can watch the same eight letters and name a different element, because they weigh the month and the rooting differently; our answer is one defensible reading with its arithmetic written down, not the only one there is.