Eight trigrams around a mirror, and two orders that do not match
A bagua mirror (八卦镜) is a small octagonal plaque, usually 10 to 15 cm across, with a mirror in the centre and the eight trigrams arranged around the rim. It is hung above a door, mirror face outward.
The ring of trigrams is almost always the Earlier Heaven (先天) sequence, the one that puts Qian 乾 at the south and Kun 坤 at the north. A compass, and every Eight Mansions calculation, uses the Later Heaven (后天) sequence instead, with Li 离 at the south and Kan 坎 at the north. The two rings look alike at a glance and put different trigrams in the same eight positions. The ring on a bagua mirror is a diagram, not a compass: read a direction off it and you get the wrong trigram.
The mirror faces out. That is the whole mechanism — it is meant to be seen by whatever is outside, not by the household. A mirror hung inside, facing the front door, is a different object doing a different job, and the two are routinely sold side by side.
What the three faces do to light, before what they are said to do to 煞
The three versions differ in geometry, and geometry is not a matter of opinion:
- The optics are checkable. Hold each one at arm's length and the size of your own reflection tells you which one you have, in a second.
- The traditional assignments are not checkable, and the sources do not agree. Some give the concave face the absorbing role, some give it the wealth-gathering role, and the roles of concave and convex are reversed in a fair share of the charts sold with the mirrors. Anyone who tells you the three faces have fixed, agreed meanings has read one supplier's packaging.
- The flat face is the one that starts arguments. It is also the cheapest to manufacture, which is why it is the most common thing hanging over a door.
| Face | What it does to the view | Traditional assignment | What a neighbour sees |
|---|---|---|---|
| Flat (平面) | Reflects at 1:1 — same size, same angle, and only for someone standing on the mirror's axis | Sends an external form back to its source | Their own front door, full size, at eye level |
| Convex (凸面) | Wider field, smaller image; the reason corner mirrors and shop security mirrors are convex | Disperses a form rather than returning it | A shrunken, curved version of the street — less confrontational, still a mirror aimed at them |
| Concave (凹面) | Narrow field, magnified image, light gathered toward the axis | Absorbs and holds a form | A bright point of light; at the right sun angle, a glare |
The tradition's scope is four shapes, narrower than the shelf implies
The Form School branch that produced the mirror dealt with 外局 — landforms and structures outside the property that the household cannot move. Four shapes come up repeatedly:
- 路冲 — a road or path running straight at the door, so that traffic and its noise arrive on axis.
- 壁刀 — the corner of a neighbouring building aimed at a window or a door, named for the edge of a blade.
- 反弓 — a road or a river curving away on the outside of a bend, so the flow turns its back on the house.
- 天斩 — a narrow gap between two tall buildings directly opposite, funnelling wind and a slice of sky at the entrance.
Measure the sightline before buying the cure
All four are sightlines as much as they are shapes, and a sightline can be measured. Two numbers decide whether an external form is worth acting on: how far off axis it sits, and how much of the view it takes up.
Off axis: stand in the doorway and look straight out. A workable convention is to treat anything within about 15° of that line as facing you, 15° to 45° as oblique, and beyond 45° as not aimed at you at all. A phone with a level or a compass app gives the angle in seconds.
How much of the view: an object subtends an angle of 2·arctan(half its width ÷ its distance). A 0.4 m roof corner 25 m away subtends about 0.9° — a hairline on the skyline, not a blade. The same corner at 4 m subtends about 5.7°. A 6 m gap between two buildings 12 m across the street subtends about 28°, which fills the doorway.
A worked case: a terraced house facing a T-junction, the road 12 m away and 6 m wide, aimed at the front door. That is 28° of road across the frame, arriving on axis, with the traffic noise to match. It is the one situation on the list where something outside genuinely dominates the entrance — and the honest fix is still not a mirror, but a screen, a hedge, or a change to which door the household uses.
The negative case is more common. A roof corner 25 m away, seen 30° off axis, subtends under 1°. It is a detail on the skyline. A mirror hung for it costs a neighbour's goodwill and buys nothing that can be measured.
What actually changes a sightline, in order of effect
If something outside is pressing on the entrance, four moves are available, and the mirror is last:
- Block the line. A hedge, a screen, a fence, or a curtain at the window removes the sightline. This works because the problem was the view, and the view is now gone.
- Change the light. A lamp at the door on a timer, so the entrance reads as lit and occupied after dark. A good share of what people describe as a heavy entrance is an unlit one.
- Change the door. Colour, a canopy, a step, a number you can read from the street. These change what the entrance is, not what stands opposite it.
- Use the other door. In a house with two, walking in through the one that does not face the road solves the problem for the household without touching the road at all.
Four questions before hanging one
These separate a considered use from a habit:
- How far off axis is it? Within 15° and 20 m, worth thinking about. Past 45°, or 50 m out, it is scenery.
- Can you change it? If the answer is yes — a hedge, a screen, a word with the building manager — the mirror is the wrong tool, because it leaves the cause exactly where it was.
- What will the neighbour see? A mirror on a shared corridor wall or a front door is aimed at a person. In an apartment block the answer is often their front door, at eye level.
- Is there a cheaper fix? A curtain, a lamp, and a screen cost less than a mirror, and all three can be undone.
What the object can and cannot do
A bagua mirror is not a charm against misfortune, and it does not attract money. It is a small mirror from a branch of feng shui that dealt with landforms outside a house — a real category with a narrow application, most of it concerned with roads, corners, and wind. The entrance to a home is improved by a light that works, a path that is clear, and a door that closes properly. None of those three needs the neighbour's consent, which is the practical difference between them and a mirror on the wall.
Disclaimer: This content is for informational and cultural reference purposes only. It does not constitute professional medical, legal, financial, or psychological advice. Readers should exercise their own judgment and consult qualified professionals for specific concerns.
Sources and review basis
Traditional references explain historical vocabulary and context. Official practical sources support only the observable safety, comfort, or environmental points noted in the article; they do not validate metaphysical claims.
Traditional reference
Huangdi Zhaijing (黄帝宅经)Wikisource digital text · Accessed 2026-07-14
Historical context for traditional residential orientation and spatial vocabulary. Modern recommendations are judged against real conditions.
Traditional reference
Book of Changes (周易)Wikisource digital text · Accessed 2026-10-14
The classical text that writes yang lines as 九 and yin lines as 六, which is where calling the ninth day double yang comes from. Cited for terminology, not for any claim about the date.
Practical source
APA Dictionary of Psychology: Barnum effectAmerican Psychological Association · Accessed 2026-07-14
A psychology reference for why broad, favourable personality descriptions can feel personally accurate. It does not evaluate BaZi calculations themselves.
