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What causes a guitar top to sink or belly?
Same villain behind most acoustic structural drama: sustained string tension, going to work on a top that's either lost some structural margin or was never quite built to fight it forever.
Bellying near the bridge
The classic tell — a slight dome or bulge in the top just behind the bridge, sometimes paired with a slight sinking in front of it. What's happening: string tension is basically trying to rotate the bridge forward and drag the top up behind it, and over years, the wood (and the bracing propping it up) can start losing that tug-of-war. A bellying top and a lifting bridge are often two symptoms of the same problem — see Bridges • How do I know if my bridge is lifting? if you're seeing both.
Why it happens more on some guitars than others
Lighter bracing (including scalloped bracing, which trades structural margin for tone on purpose — see Bracing • What is scalloped bracing?), thinner tops, older guitars where wood and glue joints have had decades to slowly creep, and guitars stuck in bad humidity are all more prone to it. Heavier string gauges and higher tunings speed the whole process up by just adding more relentless force to the equation.
Is it dangerous?
Mild bellying is common on older guitars and isn't automatically an emergency, but it's worth keeping an eye on. Action creeping up over time is usually the first thing you'll actually notice, since a bellying top quietly rewrites the geometry between your strings and the fretboard. Severe bellying can mean the bracing itself has come loose or failed, which is a much bigger structural repair.
What to do about it
Mild cases are often managed rather than "fixed" — keep humidity stable, don't go heavier on string gauge than the guitar was designed for, and watch for progression. More serious bellying may need bracing repair or reinforcement (for the full rundown on why bracing matters in the first place, see Bracing • Why do acoustic guitars need bracing?). If your action's crept up over the years and you can't explain why, this is worth ruling in or out before you blame anything else.
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