Three feet always land
Why Does a Chair Wobble When a Stool Never Does?
Three feet always sit flat on any floor at once. A fourth foot has to get lucky.
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Step 1 of 5
Any three feet can always be laid flat.
This is not good design. It is geometry, and it cannot be got wrong. Take three points anywhere and there is exactly one flat surface passing through all three. That is why a three-legged stool cannot rock: whatever the floor is doing, all three feet reach it.
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Step 2 of 5
A fourth foot arrives too late.
The first three feet settle onto the floor and use up all the freedom. The fourth is stuck with whatever height that decision leaves it, and the floor rarely agrees.
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Step 3 of 5
No floor is actually flat.
The chair is not badly made. The gap is smaller than the floor's own error. Boards cup, tiles sit proud, carpet compresses unevenly and concrete cures with a slope. A gap of half a millimetre under one foot is enough to rock a chair you can hear across a room.
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Step 4 of 5
This is why cameras stand on three legs.
Anything that must not move gets three feet. A tripod cannot rock, so it never needs adjusting once it is set down, on any ground at all.
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Step 5 of 5
A folded napkin is not a repair.
You have not fixed the chair. You have fixed one spot on the floor. Packing under one foot builds a new surface that all four happen to meet. Move the chair a few centimetres and the floor underneath is different again, so the fix travels nowhere.
The short version
Three feet always reach the floor together because three points decide a flat surface — a fourth foot has to match a surface that was settled without it.
Try it yourself
Rock a wobbling chair, then turn it a quarter turn on the spot and try again. Often the wobble goes, because a different three feet are now doing the deciding.