Truss rod adjustment is a subject many view with fear because there is a very real danger of ruining your prized instrument. Warped necks, stripped truss rod nuts or broken truss rods are a few of the results of a too-heavy hand while adjusting. However, with a little knowledge and a bit of care, there is no reason why you should do any damage. There is one simple rule: If you are not comfortable doing these adjustments yourself - DON'T. Refer it to your luthier and rest easy.

What is a truss rod?
The truss rod is, quite simply, an adjustable steel reinforcing rod inside the neck which makes it stiffer and braces it against the tension of the strings. The string tension tends to pull the neck upwards at the ends, causing a dip away from the strings (called relief) in the middle of the neck. Tightening a truss rod reduces relief and loosening increases relief.

At first thought, it might seem that a neck needs to be flat from first to last fret, but due to the way strings vibrate there actually needs to be a slight relief downwards between the 12th and 1st frets. If the neck curves up in the middle without enough relief, this is called a bow.

Truss rod myth
MYTH: The truss rod is for adjusting your action! I have personally seen this myth destroy dozens of necks in the last few years. While a truss rod adjustment can affect the action of an instrument, this is a side-effect, rather than the reason for adjusting.

Checking the relief of your guitar neck
Tune your guitar to the exact tuning you use. Only under the correct tension will the neck bend to the degree you need to adjust for, and you will get a faulty impression of the curvature of the neck if the tension is incorrect.

Hold your guitar in the playing position. Push the low E-string down on the first fret and the highest fret simultaneously and inspect the relief (gap) between the bottom of the string and the top of the 7th and 8th frets. There should be about a millimetre gap. Some guitars will need a little more or less than this. Also, if you play harder you may need more relief than someone with a light touch.

Use the right tool
Use the correct tool to do any adjustment. This will be in the form of a spanner, allen key (hex wrench), or in some cases, a screwdriver. If you don't have the correct tool, get it, as using the wrong tool may ruin your truss rod nut. Many American made guitars will have Imperial size nuts, while Asian will have metric - the two will often be similar in size, but can slip and do damage when used.

Adjusting the truss rod
Locate the truss rod nut. This will be at one end of the neck or the other (usually behind the nut, or at the base of the neck. Remove the truss rod cover if there is one.

If there is not enough relief or none at all, loosen the truss rod a quarter turn in an anticlockwise direction. If the relief is too large, gently turn the truss rod nut one quarter turn in a clockwise direction. Do not make any adjustments more than a quarter turn at a time. Check the relief again, and if it is still not right, repeat the process.

Once everything is right, put the truss rod cover back and play. Check it again in 24 hours - the truss may have "settled" a bit and need a slight adjustment to tweak it to perfection again.

Warnings
If the truss rod nut: needs large amounts of adjustment; makes loud creaking sounds; or turns with too much or no resistance, chances are something is WRONG and you should take it to a professional.

Special notes:
Double truss rods - These are very tricky to adjust but fortunately quite rare (mostly found on some basses and Rickenbacker guitars)! Don't tackle these yourself, but take the guitar to your luthier.

Some Fender guitars often have the truss rod nut at the base of the neck, hidden by the scratchplate. With these, you need to remove strings, remove the neck, adjust the truss rod nut, then put the neck back, replace the strings, retune, check relief, remove the strings, remove the neck, adjust the truss rod nut, etc. etc. etc....