# How charting works (/en/docs/guide/concepts/how-it-works)
Nine steps from birth data to a complete natal chart — what each step does and what it works from.
*For: everyone. To drive the steps through the API, see [The API behind the nine charting steps](/en/docs/guide/guides/step-api)*
Charting is not mysticism; it is a chain of determinate derivations. Given the same birth data and
the same school-of-thought choices, anyone should arrive at exactly the same chart. This page breaks
that chain into nine steps.
Everyday charting does not require walking these steps yourself — the charting entry point does all
of it. Read this page to know where the results come from, and why a particular field looks the way
it does.
## The whole flow [#the-whole-flow]
**Convert Gregorian to lunar**
— obtain the lunar year, month and day, plus the four pillars
**Fix the month index**
— resolve leap-month attribution
**Locate the Soul and Body palaces**
— from the month index and the hour
**Determine the Five Elements class**
— from the Soul palace's stem and branch
**Place Ziwei and Tianfu**
— from the Five Elements class and the lunar day
**Place the fourteen major stars**
— unfolded from the positions of Ziwei and Tianfu
**Place minor and adjective stars**
— each derived from the year stem, year branch, month, day or hour
**Place the four groups of twelve gods**
— Changsheng, Boshi, Sui-qian, Jiang-qian
**Derive decadals and age fortune**
— from the Five Elements class, gender and year branch
***
## Step by step [#step-by-step]
### 1. Convert Gregorian to lunar [#1-convert-gregorian-to-lunar]
Zi Wei Dou Shu is built on the lunar calendar, but the input is usually Gregorian. This step also
produces the stem-branch pillars for the year, month, day and hour.
When the year's stem and branch turn over is configurable: someone born between lunar New Year and
the Beginning of Spring (立春, the solar term around 4 February) gets a different year pillar under
each setting. Year-derived adjective stars take their year branch from a *separate* switch, and the
two switches can be set independently — so year-derived adjective stars and the major stars may rest
on different year branches. This is iztro's actual behaviour, reproduced verbatim by x-iztro.
### 2. Fix the month index [#2-fix-the-month-index]
Days after the 15th of a leap month count towards the following month (this can be switched off),
and the late Zi hour takes no part in the correction.
### 3. Locate the Soul and Body palaces [#3-locate-the-soul-and-body-palaces]
Start at the Yin palace as the first lunar month, count forward to the month of birth, then count
backward from there to the hour of birth — where you land is the **Soul palace**. The **Body palace**
uses the same starting point but counts the hour forward. The Soul palace's heavenly stem is derived
from the year stem by the Five Tigers rule.
### 4. Determine the Five Elements class [#4-determine-the-five-elements-class]
Looked up from the **Soul palace's stem and branch**, with five possible values: water 2nd, wood
3rd, metal 4th, earth 5th, fire 6th.
The class number (2/3/4/5/6) gets used twice further on: as the divisor when placing Ziwei, and as
the starting nominal age when deriving the decadals.
The earth plate takes the class from the Body palace's stem and branch instead, and the human plate
from the Spirit palace's. Change the stem and branch the class is taken from and steps 5, 6, 8 and 9
are all recomputed — that is what the charting-perspective configuration does.
### 5. Place Ziwei and Tianfu [#5-place-ziwei-and-tianfu]
Divide the lunar day by the class number and apply the Ziwei placement rule to fix which palace
Ziwei falls in. Tianfu mirrors Ziwei about the Yin–Shen axis: the Tianfu slot = 12 − the Ziwei slot
(modulo 12).
### 6. Place the fourteen major stars [#6-place-the-fourteen-major-stars]
The six stars of the Ziwei series unfold **counter-clockwise** from Ziwei's slot at fixed offsets,
and the eight stars of the Tianfu series unfold **clockwise** from Tianfu's slot. The offsets are
not contiguous — there are gaps:
| Ziwei series (backward, from Ziwei's slot) | Slots back |
| ------------------------------------------ | ---------- |
| Ziwei | 0 |
| Tianji | 1 |
| Taiyang | 3 |
| Wuqu | 4 |
| Tiantong | 5 |
| Lianzhen | 8 |
| Tianfu series (forward, from Tianfu's slot) | Slots forward |
| ------------------------------------------- | ------------- |
| Tianfu | 0 |
| Taiyin | 1 |
| Tanlang | 2 |
| Jumen | 3 |
| Tianxiang | 4 |
| Tianliang | 5 |
| Qisha | 6 |
| Pojun | 10 |
With both series laid out, all fourteen major star positions are fixed. The birth-year stem's
mutagen marks are stamped on in this step too.
### 7. Place minor and adjective stars [#7-place-minor-and-adjective-stars]
Each group is placed from something different:
| Group | Derived from | Example |
| ------------------------------- | ------------------------- | -------------------------------------------------------------------------- |
| Lucun, Qingyang, Tuoluo | Year stem | "jia's lu goes to the Yin palace", with Qingyang before and Tuoluo after |
| Tianma | Year branch | Only ever lands on the four horse branches (yin, shen, si, hai) |
| Tiankui, Tianyue | Year stem | |
| Zuofu, Youbi | Lunar month | "from Chen, count the months forward for Zuofu" |
| Wenchang, Wenqu | Hour branch | "from Xu, count the hours backward for Wenchang" |
| Huoxing, Lingxing | Year branch + hour branch | |
| Dikong, Dijie | Hour branch | Start at the Hai palace for the Zi hour, Dikong backward and Dijie forward |
| Santai, Bazuo, Enguang, Tiangui | Lunar day | Counted from the positions of the minor stars |
| Year-derived adjective stars | Year stem or year branch | The largest group |
All four use the same lunar day count, but start from different places and run in different
directions: Santai counts **forward** from Zuofu's slot, Bazuo **backward** from Youbi's slot,
Enguang forward from Wenchang's slot and Tiangui forward from Wenqu's slot (with Enguang and Tiangui
each then stepping back one). This is iztro's actual algorithm, reproduced verbatim by x-iztro.
### 8. Place the four groups of twelve gods [#8-place-the-four-groups-of-twelve-gods]
Each group fills all twelve palaces with twelve markers, exactly one per palace:
| Group | Origin | Direction |
| -------------------------- | -------------------------------------------------------------- | ------------------------------- |
| The twelve Changsheng gods | Fixed by the Five Elements class (water 2nd starts at Shen, …) | Gender and year-branch polarity |
| The twelve Boshi gods | The palace holding Lucun | As above |
| The twelve Sui-qian gods | The palace of the year branch | Always forward |
| The twelve Jiang-qian gods | Fixed by the year branch's trine group | Always forward |
**Sui-qian** (岁前, "ahead of the year") is a cycle of twelve annual markers counted from the year
branch itself. **Jiang-qian** (将前, "ahead of the general") is a second cycle of twelve counted from
the General star, whose position comes from the year branch's trine group. Both are per-palace
annual markers; they are unrelated to the fourteen major stars.
### 9. Derive decadals and age fortune [#9-derive-decadals-and-age-fortune]
Decadals start at the Soul palace, ten years per palace; the starting nominal age is the class
number (water 2nd starts at nominal age 2, wood 3rd at 3, …). Direction is decided by the polarity
of gender against the polarity of the year branch.
Age fortune runs on a separate scheme: it starts from the palace fixed by the year branch's trine
group and advances one palace per nominal year, with direction depending on gender alone (male
forward, female backward).
**Nominal age** (虚岁) is East Asian age reckoning: you are 1 at birth and gain a year at the turn of
the year, not on your birthday.
***
## Horoscopes are a separate line [#horoscopes-are-a-separate-line]
The nine steps above produce the **natal chart**, computed once and fixed thereafter. A horoscope
projects the natal chart onto a moment in time: the stems and branches for that level are computed
from the target date, then the twelve palace names are re-laid-out and that level's mutagens and
flowing stars are derived.
The natal chart does not change; what changes is "which palace you are standing in right now". See
[Horoscopes](/en/docs/guide/concepts/horoscope).
## Driving the steps through the API [#driving-the-steps-through-the-api]
Each of the nine steps has a corresponding public function in x-iztro, so you can take just the
result of one intermediate step. The mapping table, and which steps configuration changes, are on
[The API behind the nine charting steps](/en/docs/guide/guides/step-api).