First let's nip this very popular myth in the bud. If the neck of your guitar is indeed warped, there's very little you can do. If it's seriously bad, get rid of the guitar and get another one. If it's an heirloom or collector guitar, be prepared to spend a lot of money to have the fingerboard planed and re-fretted, and you will probably ruin some of the collectable value in doing so. But if it's warped seriously bad, it's not worth much anyway. If you tighten a truss rod too much, you will in fact permanently warp the guitar neck in an often undesirable location.

Now that we've dispensed with the formalities, what's the point of having an adjustable truss rod on a guitar anyway, if not to straighten the neck? Truss rods have evolved over time; and there are many claims that are made about them. But the primary purpose of a truss rod is to have a metal reinforcement piece inside a wooden guitar neck to counteract the strain on the wood caused by the guitar strings. A guitar neck will naturally develop a bit of a concave bow if left on its own for a long enough period of time. A very small amount of concave bow is actually desirable. But the forces exerted by steel strings on the neck usually create an unacceptable amount of bow over time. Some of the first attempts at correcting this bow are still in use today in the form of metal insert strips. That's all they are - metal reinforcement rods - and they work quite well.

Later evolution spawned the adjustable truss rod and recently the two-way adjustable truss rod. These consist of a reinforcement rod anchored at one end and with a tightening screw at the other. By tightening the screw, the tension is increased relative to the wood that it is anchored in. Increasing the tension increases the stiffness. All of this is a long way of saying that the purpose of the truss rod is for reinforcement. It is used to adjust the stiffness of the neck. Somewhere along the line somebody discovered that they could sometimes correct small variations in the neck flatness by tightening the rod - and this led to the myth. The newer two-way rod flexes in both the convex and concave dimensions depending which way you turn the screw. These devices legitimately claim to affect neck flatness, but they do so in a very limited area of the neck.

Will adjustment of the truss rod help you? Only if your guitar neck has a very well-formed natural bow (concave or convex) lying precisely halfway between the neck joint and the nut. If you've got bowing occurring precisely at the neck joint, you have other issues to deal with. If you've got two humps between the nut and neck joint, it's very likely somebody tightened the truss rod way too tight and ruined your guitar. If the neck looks like a roller coaster or is seriously bowed within the first 3-5 frets, it is doubtful that adjustment of the truss rod will have much impact.

Now that you have some background, let's say your neck isn't perfect and you want to do something about it. You open the truss rod cover plate and find the allen wrench with which to turn it. What do you do?

The first thing you should do is loosen the truss rod completely. We will assume for this article that you have a one-way rod. Loosen it until the nut spins freely and all tension is removed. Next turn it back until it is snug, finger tight. If your strings are loose, tune them up at this point. Now using the wrench, turn the nut about one full turn beyond finger tight. The purpose is to apply tension to the neck. That is all that we are doing.

Wood under tension takes time to settle. What you should do now is put your guitar in its case and leave it for about two weeks. That's right, two weeks. You can play it if you want, but don't do any more tweaking until it has had a chance to settle. In fact your guitar is properly adjusted at this point. You can just put the cover back on.

OK, you aren't happy with it and you've come back two weeks later. Put a metal straight edge across the length of the fingerboard. Look at the frets in the middle of the straight edge. If there's a gap, your neck is concave. If there's a 'hump', your neck is convex. Let's say there's a gap. Tighten the truss rod. One half of a turn at most. Leave it for two weeks.

Let's say there's a hump. Loosen the truss rod. One half of a turn at most. Leave it for two weeks.

It's been a month and it still isn't right. What do you do? You can either live with it, or find another guitar. If you're getting noticeable fret buzz when you play, you can raise the string action a bit by adjusting the height of the bridge with shims or adjustment screws. If the last truss adjustment helped a little bit, you can try it one more time. That is absolutely the last time. If you tighten it too tight, you will end up with either a) a double hump in the fingerboard, or b) a broken truss rod. If you are in doubt how tight it is, loosen it completely and start over.