You can check the alignment of your guitar's neck by holding it by the body and tilting the neck down and away from you. You particularly want to see the top edge of the neck. Figure GM-3 shows the position for viewing the neck alignment. View the neck from various angles at this position and look for bowing, twisting, or tilting of the neck.
Most guitars (from 1970 to present, at least) have a steel rod inserted in their neck which provides a means of straightening it when it becomes bowed or warped. The steel rod itself is slightly bowed and can be turned to compensate for any warping or bowing of the guitar's neck. One end of the rod protrudes beyond the end of the neck and has a hex hole or hex nut (or a slot of some kind) to which you fasten a tool (usually a hex key) to turn the rod. On some guitars the rod protrudes at the head of the guitar near the nut, while on others it protrudes where the neck meets the body of the guitar (either at the front or the back). Figure GM-4 shows the rod end protruding on two different guitars.
If your guitar neck becomes bowed it's usually due to repeatedly being exposed to drastic temperature or humidity changes, over tightened strings, careless storage (such as leaning it against a wall with the weight of the guitar supported by the head for long periods), or, poor materials or craftsmanship.
I've rarely seen a "twisted" guitar neck, but I have seen many expensive guitars with necks that bowed after being neglected for a long period. By "twisted" I mean, well, imagine placing one hand on the head of your guitar and the other hand on the body and then turning your hands in the opposite directions, like wringing out a mop. If you were strong enough you could twist the neck. DON'T TRY IT! YOU MIGHT BE STRONG ENOUGH TO BREAK IT! Twisting doesn't happen often because the force exerted by the tension of the strings tends to prevent it. However, that same steady force can cause the neck to bend upward from the nut end of the neck and that causes the strings to be farther away from the fretboard. The neck can also become bowed in the opposite direction, which makes the strings sit against some frets and causes them to buzz when played below those points on the fretboard, but, that symptom can also be caused by the neck being tilted too much in relation to the body. Aligning the tilt of the neck will be discussed later.
To straighten the neck you must turn the metal rod until the neck is correctly aligned again. I suggest you use a straight metal-edged ruler, 12 to 24 inches long (if you can find one) to check for any bowing. If the ruler rocks back and forth when placed length-wise along the fretboard, then the neck has bowed downward at the nut end. If there is any space below the ruler's edge and any of the frets, then the neck has bowed upward at the nut end. Determine what type of tool you need to turn the rod (It will probably be a hex key, but the neck rod on some guitars require a screwdriver for making adjustments). Turn the rod in either direction 1/4 turn and check the ruler again to see if the neck is straighter. If it is worse, turn the rod 1 half turn in the opposite direction, otherwise, turn it slowly in the same direction while you keep checking the neck with your ruler. Remember; turn slowly once you've determined that the direction of the turning is correct (the neck is bowed less). View the neck as shown in Figure 12-1 again for an overall assessment of how straight it is after making the adjustments and re-adjust it as described above if necessary.
As mentioned above, if the neck is tilted it is not aligned straight with the body. If the neck is tilted down it causes your strings to buzz. If it is tilted up, it causes them to be further from the fretboard and makes playing more difficult. Both conditions can also cause the intonation of your frets to be set incorrectly. We'll discuss intonation later.
After checking to see if your neck is bowed or not and correcting it if necessary, you should check it's tilt. You can check it by holding the guitar edge-wise and viewing the body and neck to see if they are parallel. You can check more accurately by placing one end of a piece of thread against the body where it meets the neck and extending the other end down the neck and hold it beside the nut. The distance from the thread to the face of the fretboard (where your place your fingers to play) should be uniform along the entire length of the neck. Make sure the thread is sitting flat on the body of the guitar and does not bend as it extends beyond it.
Unfortunately, not all guitars provide a means for adjusting the tilt of the neck, so, if yours doesn't, all you can really do is to tighten the screws that hold the neck to the body. If your guitar does have special screws for adjusting tilt, you should read the owner's manual for details about the procedure for making adjustments. Aligning the tilt of the neck is usually done by turning a small hex screw, which is accessible through the back of the mounting brace. You should be aware that it is probably necessary for you to loosen one or more of the main neck-mounting screws before adjusting the alignment screw.
Some guitars have adjustable bridges and some don't. Acoustic guitars usually don't. They often use "shims" (thin wedges of plastic or bone) which are placed beneath the main bridge piece (the part that is in contact with the strings) to raise it. You would remove the shims to lower the bridge, of course.
Electric guitars come with one of several different bridge designs. Some have a solid bridge that is raised or lower by adjusting screws at each end, while others have individual bridges for each string. Obviously, a solid bridge is easier to adjust, but it can't provide the accuracy for all strings like individual bridges can. Not only is an individual bridge more accurate in raising or lowering each string, it also allows you to move the string forward or back to adjust the intonation of an individual string. We will discuss intonation adjustments below. Figure GM-5 shows a commonly used bridge design. The arrows indicate the individual bridge for each string.
Ideally, you want the bridge(s) to be high enough that the strings will not vibrate against the frets when you are playing anywhere on the fretboard, and, you don't want the bridge set too high because it will make pressing the strings against the fretboard more difficult.
Almost all bridges are raised and lowered by turning 2 or more screws. Some guitars have hex screws on the bridge and others have slotted screws. Be sure to use the correct size screw driver or hex key for your bridge. Otherwise, you might cause damage and you won't be able to make any adjustments.
Before adjusting the bridge, make sure you have checked the neck for bowing and warping, and also check the tilt. If the neck is straight, play every fret on each string one at a time, all of the way up the neck until you reach the body of the guitar. Listen carefully as you do so, and if any notes cause a buzzing sound; adjust the bridge by raising it very slightly. If your bridge is one piece for all strings, then raise the half of the bridge that supports that string. For example if your D (3rd) string buzzes, raise the screw at the top end of the bridge because the D string is on the upper half of it. Make small adjustments until the buzz is gone. Check every fret again to be sure. If you have individual bridges for each string, then obviously you would only raise the bridge for that string. On the other hand, if you think the bridge is too high, then lower it very slightly while you continuously check the string for buzzing. When the string starts to buzz in any fret(s), raise it again until there is no buzzing in any of the frets.