When a guitar is new the neck should be adjusted from the factory to be relatively flat, but over time string tension, humidity, and other factors can cause the guitar neck to bow out of adjustment. Adjusting a guitar neck is done by turning the truss rod that runs down the interior of the neck beneath the fret board. This can reduce excessive forward bow, referred to as relief.
Before adjusting the neck of your guitar it is important to note that many novices mistakenly assume that if the string action is too high, a truss adjustment will fix things. Due to the way strings vibrate when strummed or plucked, some amount of forward relief is normally required in a guitar neck to keep strings from buzzing against frets. If a truss adjustment is unnecessary, adjusting the neck into a backward bow to lower action will make the guitar sound worse, not better.
Though a standard formula would be nice, the ideal amount of relief in a guitar neck varies from instrument to instrument and depends on many factors including string gauge and style of playing. Check the relief before making any adjustments. If it appears to be good, yet the action is high or the guitar buzzes, it might need a nut, saddle or fret adjustment instead. If the neck shows excessive bow, continue with the truss rod adjustment.
The truss rod adjustment head can be found in different places depending on the guitar model. On electric guitars it is either under the pickguard or at the base of the headstock and might require removing the neck. On steel-string acoustic guitars it is at the base of the headstock or accessible through the sound hole. When located on the headstock a small plastic or wooden plate held in place by screws sometimes covers the truss rod. Many 12-string guitars and bass guitars have double truss rods that are best adjusted by a professional.
Truss rod heads are designed differently and in some cases before you can adjust the guitar neck you’ll have to get the right tool. Some rods have a hex nut welded to the top. Many guitars including Martins and Fenders® require an Allen wrench, while Gibsons® and Taylors® require a nut socket. Use the correct size wrench or socket to avoid damage to the truss rod head.
Before starting the adjustment mark the truss head's position. This will not only allow you to see where you started, but you can return to the original setting if desired. Be sure to adjust the guitar neck with all strings in place and tuned to pitch.
Tightening a truss rod (turning it clockwise) increases back bow of the neck. Loosening it (turning it counterclockwise) increases forward bow. A typical adjustment requires significantly less than a quarter of a turn of the truss head.
No matter what kind of relief you seek, always start by loosening the truss a small degree, turning it very slowly. This should not take tremendous torque. If the truss doesn’t budge with moderate effort, you might be better off taking the guitar to a shop rather than risking breaking the rod.
Providing the truss turns easily enough, slowly make your adjustments as needed. Turn the truss in small increments and check the relief between adjustments, giving the guitar neck time to settle in.
If the truss turns easily but doesn’t seem to be affecting relief of the neck, it's best to take it to a shop for advice. Truss adjustments are very inexpensive so the only reason to do it yourself lies in the pleasure of caring for your instrument(s) personally.