Whenever possible when inspecting a guitar, we string it up and check how it plays. We want to know how the neck responds to string tension, and, critically, we want to know if it has developed any undesirable distortions in its geometry.
Here is a box-fresh guitar. Nothing has moved and you can see we’d have no trouble playing this any where up the neck. Often, though, acoustics will develop a ‘belly’ over the years due, principally, to the stresses of high tension strings. A slight belly is fine (and desirable, in fact) but on a guitar, just as on a guitar repairer, a big belly is bad news.
Swollen tums increase string height and make the guitar hard to play. And these problems can be exacerbated by contrary neck movement and the dipping of the fingerboard extension: essentially, the guitar folds in on itself.
3. Ugly: Top with belly + adverse neck movement
When this happens, steps need to be taken to prevent further insidious movement and to bring strings back down to a sensible height so they run, more or less, parallel to the fretboard. One standard way of achieving this is by removing the neck and adjusting its angle: the neck reset.
4. After a neck reset