Just about every steel string guitar made these days has a truss rod inside the neck. Most often the rod is placed down the center of the neck, before the finger board is glued into place. The purpose of this steel rod is to counter the pull of the strings on the neck.
Imagine the neck like a bow, the more the strings are tightened, the more the neck will bow. The rod prevents this by applying counter force within the neck.
Every guitar is different but typically your truss rod will need adjustment when changing tuning, changing string gauge, or subject to environmental/seasonal/humidity changes. Example winter to spring, summer to fall, etc. Any time your guitar's playability changes after it's been properly setup (for a certain tuning and string gauge) it is due to the neck needing a re-adjustment.
Generally, although there are exceptions, when the neck is setup right for you, it will not need more than a quarter turn in either direction to compensate for seasonal changes- this can change a buzzy neck to near perfect action again, or that from uncomfortably high string action back to how it was when it first left the shop. Most players agree, a near straight neck is the most comfortable to play on.
There are a few golden rules when adjusting your truss rod. First, only use the rod to keep your neck as straight as it needs to be…do not use it to adjust your action! Second, only use the proper tool. If you do not know what it is, check with the manufacturer. Truss rod nuts are easy to strip, and once they're stripped, expense to fix! Easily costing more than the guitar is worth. Finally, do not force anything; an eighth of a turn can make a drastic change. Remember, if the truss rod feels tight, take it to a professional.
If you feel the neck needs an adjustment, read below. The procedure below list how to ACCURATELY adjust the truss rod, which affects the relief in the neck. If you don't follow the outline, your results will not be measurable or accurate.
What you'll need:
How much to adjust:
The bigger the gap, the more relief in the neck. We are usually wanting just a small amount of relief in the neck for optimal playing. Some players like the way a dead straight neck feels, and this will produce fret buzz, but those players live with it.
If there is a gap, tighten the truss rod until you have a gap about the size of a standard business card, string package, or even a piece of paper for minimal relief. Go ahead and slide it in between the top of fret and bottom of string (or ruler, whichever method you are using) at your half way mark, making sure you are placing the card parallel with the string or ruler.
To get technical, you can start with around .008”-.010” for most electrics, .010”-.012” for acoustics, and .014”-.016” for bass (this is where an automotive feeler gauge comes in handy).
If there is no gap at the 6th fret, you are either dead straight or back bowed away from the pull of the strings (loosen truss rod).
If the strings are buzzing when played open, as well as the first few frets, it is a clear indication of a back bow (loosen truss rod). At this point you may easily see by sighting down the neck (from the headstock looking down to the body) that it is back-bowed.
When a string is plucked, strummed, or picked, it vibrates in an elliptical pattern. This is why we need a slight bow (relief) in the neck to prevent buzzing, mainly in the lower register of the fretboard.
If you play hard on the strings, you will exaggerate that movement of the string and depending on preference, may need more bow in the neck, and/or a higher action.
Players with a lighter touch often have straighter necks with lower action.
This is why electrics may have less relief than acoustics (although not as a rule).
If you play bass, you will want more bow than on a guitar (the strings are that much bigger and vibrate in relativity to their size).
Please Take Note!
Straightening the neck may make your guitar play worse! Consequently, when the truss rod is tightened, the height of the strings are lowered, which can create string buzz if your guitar isn't setup properly. String height/ playing action is controlled at the nut and saddle, not in the neck adjustment! And there is a reason for that!
Likewise, tightening the truss rod may also make a poor fret condition more noticeable. If your guitar plays worse after adjusting the neck, it is either too straight for your playing style or it is a sign that more work is needed. Bring it to a professional, or go on to step 2: How to setup your guitar like a pro. A well set neck is the first step in a proper set-up which will help to make your guitar play better.