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What is the "truss rod" on a guitar? Do truss rods vary for an acoustic, electric, or classical guitar?

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A neck is not unlike a bow (like bow-and-arrow). Like a bow-string, guitar strings bend the neck and make it arc. Obviously, you don't want the neck to bend as much as a bow, or else you'd never be able to play it. But you actually want the neck to have a little bend in it, or else the strings will lie flat on the frets and won't be able to vibrate freely. The amount of bend your neck has is called it's relief.

If the strings don't exert too much tension, then they'll only bend the neck a little bit, and everything will be fine. This is the case with classical and nylon-string guitars. But steel strings exert much much more tension than nylon strings, and so they will actually bend the neck considerably—so much so that the guitar would become unplayable. That's there the truss rod comes in. It's a steel rod imbedded inside the neck. It counteracts some of the tension of the strings and keeps the neck from bending too much.

The great thing about most truss rods that the amount of tension they absorb is adjustable, so that you can actually adjust how much relief your neck has by increasing or decreasing the tension on the truss rod. This allows for very precise adjustments to the action of the guitar, making it very easy to play.

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Perfect answer in most respects, except that you don't want the neck to bow at all. The bridge and the nut hold the stings away from the frets. You want the distance from fretboard to string to be constant along the whole length of the fretboard. – slim May 28 '13 at 9:24
@slim I guess everyone's preferences differ when it comes to action, but typically the neck has a slight bow to it. You can check this out by placing a capo at the first fret and holding down the E-string at the, say, 15th fret. The string acts as a straightedge, and you can see how much bow your neck has by how high the string clears the frets in between. – Alex Basson May 28 '13 at 10:33

A truss rod stabilizes the neck of the guitar. Depending on the type of guitar or banjo, they are made from different materials and are located in different places on the instrument. See here for more info:

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To answer the unanswered part of the question, yes, they vary from guitar to guitar. Almost every steel-strung instrument has a truss rod. But some classical guitars, with the lower string tension associated with nylon strings, have no truss rod at all.

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To muddy the waters more, some makers like Rickenbacker put in two truss rods so you can combat neck curve with the truss rod as well. And I've heard of makers to put non-adjustable carbon fiber rods on both sides of the truss rod to fight next twisting. – Dave Jacoby Feb 28 '11 at 5:24

In essence the truss rod is a means of applying a force to the instrument's neck to counteract the forces exerted by the tension of tuned strings. Steel strung instruments in particular have very high tension when tuned and can easily distort the neck material if it is not reinforced.

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