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Truss Rods... oh what a relief they are! 

There seems to be a lot of misinformation on what a truss rod is or what it does. On one end of the spectrum there are those who see the truss rod as some mysterious thing that if someone other than a trained technician touches it, the guitar is likely to blow up in his face. At the other end are those that see it as some master adjustment device allowing you to dial in your preferred action just by turning it one way or the other.

The truss rod adjusts the relief (curvature) of the neck. That's it. It's not your action adjustor, although it does have an effect on action, which I will discuss later. A little bit of relief is usually needed to avoid string buzz. (see Figure A) To check relief, tune your guitar to pitch, then hold fret the string at the first and last fret simultaneously (a capo is handy). Check the space between the string and fret midway between the first and last fret (usually around the 8th fret). That space is your relief, and should be about .01" which is about the thickness of a business card. If the string is touching the fret, you have zero relief, and possibly even back bow.

Gibson introduced adjustable truss rods in 1921, and most modern steel string guitars have them. There are a number of variations. (see Figure B) Gibson uses a traditional single action rod that is laid in a curved channel under the fretboard. Most other makers use modern rods consisting of dual rods that are laid in a straight channel. Some dual rod designs are single action, meaning they can exert pressure in only one direction. Two way or dual action rods can exert pressure in both directions. Since string pressure pulls the neck up naturally, single action to pull it straight is usually all that is needed.

While the purpose of a truss rod is not to set action, it does have an effect on action. When I do a set-up, I check and adjust relief first, then set the desired action by adjusting at the nut and the saddle. Some guitars have the truss adjustment at the headstock end, and some at the heel. Warmoth uses a side adjustment mechanism near the heel for some of their necks. Many acoustics, like Martins, have it a the heel, accessible through the sound hole. Make sure to use the right wrench. Gibsons use a 5/16" nut driver, while most others use 1/8", 4mm, or 5mm allan wrenches.
Turning clockwise (usually) tightens the truss rod, thus reducing the relief. If you want more relief, turn counter-clockwise. Do only a quarter turn at a time, recheck and repeat as necessary. If it turns very hard or not at all, don't force it. A broken truss rod can be a costly repair. If your adjustments seem to be taking things in the wrong direction, stop and consult with a tech.

When might you need to adjust your relief? Guitars are made of wood, which is quite sensitive to climate changes, expanding and contracting to changes in humidity and temperature. So changes of seasons, going from one building to anther, use of air-conditioners and furnaces can all cause the wood to shift, requiring adjustments. Other things might include different tunings, strings, or anything that changes string tension. The best advice is to keep your guitars properly humidified (45-60%), and in as stable of environment as possible.