What it is: two numbers, one table
Eight Mansions (Ba Zhai, 八宅) is the smallest complete system in feng shui. It needs two trigrams and nothing else. The gua of the person — the ming gua — is derived from the birth year and gender, with the year adjusted if the birthday falls before that year’s lunar new year. The gua of the house comes from its sitting direction, measured with a compass rather than assumed. Put the two side by side and the tradition hands back a fixed table: eight directions, each with a star already attached. There is no chart to draw, no period to look up, no star to fly. That smallness is the point, and it is also why the system gets claimed for far more than it does.
The eight directions and what each one is for
The table always produces the same eight names. Four are read as favourable — Sheng Qi (生气), Tian Yi (天医), Yan Nian (延年), Fu Wei (伏位). Four are read as unfavourable — Jue Ming (绝命), Wu Gui (五鬼), Liu Sha (六煞), Huo Hai (祸害). Which direction receives which name depends entirely on the gua you started from, and the assignment is fixed by the Da You Nian song (大游年歌), a mnemonic that generates the whole table from a line of verse. Nothing in that process looks at your house, your health, or your year. It is arithmetic on a birth year and a compass bearing. Read the eight names as a vocabulary for ranking directions, and the system stops pretending to know things it was never built to know.
East group or west group: which one are you
The eight gua split into two sets. Kan 1, Li 9, Zhen 3 and Xun 4 form the east group; Kun 2, Dui 7, Qian 6 and Gen 8 form the west group. An east-group person’s four favourable directions are the east-group directions, and the same holds on the west side. From that comes a claim you will hear often: east-group people should live in east-group houses. The underlying rule is real, but the veto is not. In a city flat you rarely choose the sitting direction, and a household of four frequently contains both groups. What the rule can honestly do is rank the rooms inside the house you already have.
The three places that matter: door, bedroom, stove
Classical Ba Zhai is not a room-by-room audit. It reads three points — the door (门), the main bedroom (主) and the stove (灶) — and asks whether each sits in a favourable or unfavourable direction for the people who use it. The reason is historical. In the houses the system was built for, those three points were where household life actually happened, and surveying every square metre would have been wasted effort. Modern homes have more rooms and different routines, so keep the three-point habit as a way of setting priorities, then add the two or three rooms you genuinely spend hours in, and treat the rest as optional.
When it doesn't help
Three limits are worth stating plainly. First, it ignores form: the road outside, the shape of the plot, a bedroom with no window. A favourable direction on a bad room is still a bad room. Second, it has no time dimension — unlike the flying star systems, Ba Zhai does not shift with the period or the year, so it cannot say anything about when. Third, the compass reading is the weak link: a phone compass, a steel door frame or a five-degree error can push a direction into the next sector and rename every entry in the table. Use Eight Mansions as a tie-breaker between layouts that already work, never as the reason to accept a layout that does not.