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A guitar neck may look flat at first glance, but a closer inspection will reveal that most necks have a slight forward bow or relief to them. This tiny amount of curvature keeps strings from buzzing against fret tops, particularly when chorded. A truss rod runs internally through the length of the neck to help maintain a comfortable degree of bow. When the truss rod is adjusted correctly and the rest of the guitar is set up properly, it should be a pleasure to play.
It’s a good idea to measure neck relief from time to time, especially if you’ve noticed the guitar has become more difficult to play. It could be that the degree of bowing has increased. This can occur as a result of changing string gauges, exposing the instrument to extreme weather or humidity, or as a result of sheer time. If the neck needs adjustment, it’s easy enough to do. If the neck is not in need of adjustment but the guitar is having problems, you can look elsewhere for the culprit.
To measure neck relief, you’ll need a feeler gauge and a capo. Place a capo on the first fret, then use your finger to fret the E-string (uppermost string) closest to where the neck deepens to join the body. If you have two capos that’s even better as you can keep both hands free. The fretted string will create a straight line or plane, leaving a tiny gap over the fret tops. This gap reveals the degree of bow.
Eye the gap to find the largest space between the top of a fret and the bottom of the E-string. It will likely be about halfway down the neck at the fifth or sixth fret. Slip a feeler gauge into the gap to measure it.
There is no single measurement that represents ideal relief for all guitars. The most efficient degree of bow varies among individual instruments, and is also partly determined by strumming or picking styles and string choice. That said, a general guideline for jazz enthusiasts or those who enjoy fast, light picking might be a gap of 0.004 to 0.006 inches (0.102 - 0.152mm). Heavier strummers, rockers, or those who like lighter gauge strings will probably be happier with a gap that falls between 0.007 and 0.012 inches (0.178 - 0.305mm) to avoid unwanted string buzz.
If the bow gap is excessive, the truss rod can be adjusted to reduce relief and return the guitar to optimum playing condition. If there is no gap at all (i.e. the string lays on the fret tops), the neck is either dead flat or back bowed. A truss rod adjustment can also help in this case. If the rod is already adjusted out, switching to a heavier gauge string might help.
If you decide to adjust the truss rod, re-measure neck bow between each adjustment to avoid over-correcting. Note too that although a poorly adjusted neck can cause string buzz, you might also get buzzing with a perfectly adjusted neck. This can occur if the problem lies in a worn nut, for example, that is allowing the action (strings) to sit too low. Finally, measuring the degree of relief should not be confused with determining the action, or how high the strings sit off the fretboard when unchorded.
I own a nylon string acoustic guitar and I find even high-gauge strings sound better with a high degree of bow, if you've got the finger strength to make for it. It might not make a difference if you're finger-picking, but it's never good when you start strumming hard only to hear that dreaded fret buzz.