Most confusion about Qi Men Dun Jia comes from mixing up charts that are cast on different time scales. This page explains what the hour chart is, how it differs from day, month and year charts, why questions are read from the hour chart, and how this site generates one.
A Qi Men Dun Jia chart is a snapshot of a moment: the moment's stems and branches, the solar term, the yin or yang cycle and the structure number decide where the nine stars, eight gates, eight deities and the heaven and earth stems fall across the nine palaces. The snapshot can be taken at four scales. The hour chart (時盤) changes every Chinese double-hour, so twelve charts a day. The day chart (日盤) changes once a day, the month chart once a month, the year chart once a year.
Traditionally the coarser charts are used for planning: a year or month chart to pick a favourable direction for a move, a day chart to pick a day. The hour chart is the one used for questions: you ask about a specific matter, the chart of that moment is cast, and the matter is read from it. This is the “one question, one chart” practice, and it is what this site does.
The hour chart is the only one fine enough to be tied to the act of asking. Two people asking different questions an hour apart get different charts; the same person asking the same question the next day gets a different chart again, and that is intended: the reading belongs to the moment of asking, not to the person. It is also why the hour chart needs no birth data. Your birth year can be added as an extra reference (the “year fate”), but the chart stands without it.
True solar time first. Clock time is standard-zone time; the chart wants the sun's real position at your longitude. Between the longitude offset and the equation of time the difference can reach twenty-odd minutes, and near a double-hour boundary that is the difference between two different charts. The site converts using only your longitude — no precise location is stored.
Then the fixed procedure. Four pillars from the moment; yang cycle from the winter solstice, yin cycle from the summer solstice; upper, middle and lower period and the structure number by the Chai-Bu rule (the alternative Zhi-Run rule shifts whole blocks around the solar terms and gives a different number for a few days each fortnight); the earth plate of three nobles and six instruments; the duty star follows the hour stem and the duty gate follows the hour branch; then the rotating plate turns stars, gates and deities into place as one block (the “flying plate” school places them palace by palace instead). Every step is deterministic, which is why it can be checked: this site's charts match 297 published master charts cell for cell, 11,868 cells.
Almost always a school difference, not an error: rotating plate versus flying plate, Chai-Bu versus Zhi-Run, hour-based versus day-based casting, and the separate “yin plate” system. This site implements one school only — hour chart, rotating plate, Chai-Bu — and does not mix rules from others. If your teacher uses another school, follow your teacher.
Three question shapes cover most of what people ask. Will it work out? Find the palace of the matter's useful god, check whether its gate is open, whether the star is in season, whether the palace is empty or entombed, and how it relates to your own palace (the day stem). Where is it? The useful god's palace is the direction: Kan north, Gen northeast, Zhen east, Xun southeast, Li south, Kun southwest, Dui west, Qian northwest; the palace's imagery says what kind of place. When? The two timing rules that survived back-testing against master cases: the days that fill an empty palace (the two branches of the void), and the days that clash the useful god's palace.
Every term used here is explained in plain language in the glossary; the beginner's guide walks through a chart step by step.
No. The hour chart is cast for the moment you ask, not for your birth. Adding your birth year gives one extra reference layer, but the chart and the reading work without it.
An hour chart changes every Chinese double-hour and is used to answer a specific question asked at that moment. A day chart changes daily and is traditionally used for planning, such as choosing a day or a direction. This site casts hour charts.
Different schools cast different charts: rotating plate versus flying plate, Chai-Bu versus Zhi-Run for the structure number, hour-based versus day-based casting. This site uses the hour-based rotating plate with the Chai-Bu rule, verified against 297 published master charts.
Yes. Casting, per-palace meanings and pattern detection are free forever with no account. Only the AI reading is metered; signing in gives you two free readings.