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A truss rod is a rigid shank that runs down the length of a guitar neck under the fret board. It can be made of wood or graphite, but is more commonly made of steel. This inner rod helps counterbalance string tension on the neck, keeping the neck of the guitar relatively flat. Some guitars, such as Rickenbackers, have dual truss rods for extra stability.
Truss rods are adjustable so that neck bow (known as "relief") that might occur over time can be countered. Seasonal changes, humidity and switching between heavier and lighter gauge strings can all affect the guitar neck. The truss can only be adjusted from one location that varies on different models of guitars. In most cases the adjustment head is hidden behind a small plate, but on other guitars it is readily visible.
Electric guitars with bolt-on necks generally feature the truss rod adjuster at the base of the neck hidden under the pickguard plate. Models with this setup include older Fenders® and Fender-style copies, many Yamaha® guitars, and other models including vintage re-issues.
Newer Fender guitars place the truss rod adjuster at the base of the headstock just above the nut. The headstock is the portion of the guitar where the tuners or keys are located, and the nut is the small piece that guides the strings off the headstock. The truss rod adjuster is uncovered on these models, but recessed (as always) in a concave hollow.
Set-neck guitars, or guitars whose necks are glued on with adhesive rather than bolted on, feature the truss rod adjuster in the same location as newer Fenders. In the set-neck models, however, the adjuster is covered with a small plate fastened by screws. Gibson® and Epiphone® guitars fall into this category.
There are two different locations possible for the truss rod adjuster on steel-stringed acoustic guitars. It will either be located at the base of the headstock as noted above, or accessible through the sound hole on the inside heel of the neck. In the former case the truss rod will be covered with a plate, but if located in the sound hole, it will be readily visible.
While electric and steel string acoustic guitars are built with truss rods, classical guitars don’t require them. These guitars use nylon strings that don’t exert the same degree of tension on the necks as steel-stringed guitars, making reinforcement unnecessary.
While truss rod adjustment is not terribly complicated, doing it incorrectly can be an expensive lesson and can ruin your guitar. If you aren’t comfortable adjusting the neck of your guitar yourself, don’t hesitate to take it to a shop. Professional truss rod adjustment is an inexpensive service.
@Melonlity -- there is at least one good way to reduce tension on your guitar neck. Unless you are playing it, tune it down about a step (in other words, tune the bottom string (the thickest one) from "E" to "D" and back all the other ones off a step, too). That will create less tension and make your guitar neck stay in its intended shape longer.
I picked that tip up after I got a 12-string guitar. If you want to talk about an instrument that is subject to too much string tension, a 12-string is it.
Most acoustic guitars have tuners built in these days and most electric guitar amps have tuners, so it is easy to tune up properly when you pull your guitar out to play it. It's not even that inconvenient to tune your guitar down before you slap it back in the case and can extend the life of the neck. Simple solution, huh?
I agree that you should keep your hands off of that truss rod unless you know what you are doing. I've seen more than one guitar ruined by someone who got too aggressive with a truss rod. If you don't ruin your guitar neck, you can still make it so that the instrument is almost impossible to tune, the action is too low or high and all sorts of bad things can develop.
Here's a question, though. Is there a way to reduce that tension so can make sure a truss road adjustment is a long way off?
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