Several models of acoustic and electric guitars include an Allen wrench with purchase. This tool is to aid in turning a special nut that adjusts a part of the guitar called the truss rod. Many novice players are hesitant to change or otherwise adjust this, as it affects the bend of the neck. With special care and attention, however, this device can be used to better adapt the instrument to your playing style.
The truss rod is a simple device that counters the pull of the strings, controlling how much the guitar's neck can bow and flex. It does not adjust the height of your strings or the intonation. It utilizes a steel rod with an anchor at one end of the neck and an adjustment nut at the other. Some models of guitars feature two-way rods, ones that can move a neck both toward and away from string pull, but most instruments use the simple one-way rod.
While the rod itself is often inlaid (usually in a curve) into the center of a neck, the nut to adjust it may be located in any number of places in the guitar, depending on make and model. If the nut is at the peghead, it will be concealed behind an easily accessible small truss rod cover. Nuts at the heel end of the neck can be adjusted through a hole just beneath the fingerboard extension and through the top brace. In either case, a small Allen wrench may be used to turn the nut.
Tightening the adjustment nut pulls the rod straight, thus straightening the neck. Loosening it allows the neck to move with the influence of the strings, allowing the neck to bow. Tightening the truss rod straightens the neck and consequently lowers the strings, which can create string buzz. Having a straight neck in general results in a better playing instrument, but not all guitars and playing styles allow for a completely straight neck. Therefore, some relief is acceptable, and often necessary, to prevent buzzing. There is no single amount of neck relief that will be correct for all acoustic guitars. Rather, it can vary based on string gauge and playing style. If you tend to be an aggressive player, you may need some relief and a higher action. Players with a lighter touch benefit from straighter necks and lower action. Electric guitars usually have straighter necks than acoustics, and basses typically have more relief than guitars.
Before you adjust the nut, make a mark on it that corresponds to a fixed point below it on the access channel to the nut. This will help you gauge how much you have turned and can help you return to the original setting in case of error. Tune the guitar to pitch, then either fit a capo to the first fret or fret it by hand. Fret the string at the last fret as well. Measure the gap between the bottom of the string and the top of the seventh fret. A large gap here means the neck has too much relief and the rod may require tightening to straighten the neck. If there is no gap, the neck may be dead flat or "backbowed," which may require loosening the truss rod and adding relief. Locate the adjustment nut, then carefully turn it either to the right (to tighten) or to the left (to loosen) with an Allen wrench in increments no larger than one eighth of a turn at a time. If you encounter resistance, do not force it.
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