A rocking chair is usually not a short leg — and the test that tells you takes thirty seconds on a hard floor. Carpet hides the difference, then you spend an hour fixing the wrong thing.

Three causes that look identical when you sit down

A wobbly chair has exactly three common causes, and they need three completely different repairs. Using the wrong one doesn't just fail — it makes the next attempt harder:

• Loose fasteners. The screws and bolts holding the seat to the legs, or the corner blocks underneath, back out over years. Tighten them in a diagonal pattern like wheel nuts — a small turn each, working around the chair — so the frame pulls up square instead of skewed towards whichever corner you tightened first.
• One leg short, or a floor that isn't flat. This is why the test has to happen on tile. On carpet, a chair with a perfect frame rocks anyway. Cast a card under the lifted leg until it stops rocking, measure that stack, and match it with a felt pad — 2–3 mm covers most cases.
• A failed glued joint. The glue in a mortise or dowel joint crystallises after years of use. Squeezing new glue into the gap is the classic mistake: fresh glue won't bond to old residue, so it holds for weeks and fails. The joint has to come apart, the old glue scraped back to bare wood, then re-glued and clamped for 24 hours.

What makes it worse

• Sawing a leg. Without a jig you will cut at an angle, and now you have a shorter chair that still rocks. Build the short leg up instead.
• Steel brackets or screws driven through a joint. They hold for a season, then concentrate all the stress in one spot and split the timber. On a nice chair, that's the end of it.
• Over-tightening. Softwood threads strip long before the screw feels properly tight. Snug is the target.
• Felt pads on a chair with loose joints. You have hidden the wobble, not fixed it — and the joint keeps working looser underneath.

Stop and call someone if the wood itself is cracked, a leg has broken through, several joints have failed at once, or a previous repair keeps coming back. Those are structural, and a chair that fails under someone is a fall.