Once you have set your guitar's truss rod and set the bridge action, it is time to look at the nut action. Nut action is essential to a comfortable action and can make a big difference to the feel of a guitar most of the way down the neck. Stock nut heights usually err on the side of caution to accommodate the widest range of players, and can almost always be vastly improved.

What is a nut?
The nut is the "comblike" affair at the top of the neck between the headstock and the first fret. Often made from plastic, bone or other mateials, and usually white, cream or black. It sets the string spacing and action at the top of the neck and is the starting point for the strings (while the bridge saddles are the ending point).

Check first
First of all, make sure if your guitar needs adjustment or not. If the nut is too low, it will be self evident by the string buzz occurring when the strings are open (unfretted), but not on the fretted notes. If it is too high, the action will be stiff at the first position.

Bear in mind there is no universally correct dimension for string action. Instead, it is necessary to set the string height at the nut based upon both theory and the particular preferences of the player. Some players prefer a stiffer feel and in these cases it's better to leave the slots a little shallower, with the strings a little higher off the fingerboard than a "perfect" low action. Speaking "perfect" low action - read on...

Measuring nut action
Here you need a set of feeler gauges (the thin metal strips that are used for gauging the size of a gap. These can be obtained from any auto-supply or hardware store.

Fret the string at the first fret and slide a feeler gauge between the string and the fingerboard, right in front (body-side) of the fret. If the feeler gauge is loose, try progressively thicker gauges, until a snug fit is found. Not too tight however - the gauge can push up the string slightly, but not too much. You may need to use a combination of different gauges to get the exact height.

When you have the correct gauge, this should be a near perfect indication of how high the string is from the fretboard at the first fret when fretted. This should be almost identical to the height of the fret and also the perfect height for the nut slots.

Cutting the nut
If you found that the string height at the nut was too high, you need to cut the grooves deeper. The tools necessary for this are either a set of calibrated nut files or a set of needle files (sometimes called key or watchmakers' files). Nut files are special files that are manufactured to cut a round-bottomed slot of a particular width. While nut files are easy to work with and remove much of the guess work from nut slot filing, they are expensive, and not essential.

Loosen the first string slightly (only the first - you need to keep as much of the normal tension on the neck as possible), so that you can lift the string out of its slot and slide it towards the second string, letting the first string rest on the top of the nut. Place the end of the gauge(s) (the ones you found to be the same height as the string at the first fret, remember?) on top of the fingerboard so that the edge of the gages are touching the nut. Just insert enough to reach to the second string.

Using either a nut file or a needle file, deepen the slot until the groove is just above the top of the feeler gauge. The slot should be cut by filing the slot at a slight downward angle from the fingerboard towards the headstock. This slight angle makes sure that the string leaves the nut at the front. Failure to cut at an angle can result in poor intonation and string buzz.

Just before the file reaches the feeler gauge, replace the string in the slot and check that it is not buzzing against the fret - remember to retune the string, or the low tension can make it buzz anyway. If it's your first time cutting a nut, it might be a good idea to stop here - it's probably already improved the action no end and it's easy to get carried away and cut too far - I still do it occasionally! Leaving a little "wiggle room" before you set the bridge action is also a good idea. After all, it's easy to go back and tweak the nut action later once everything else is adjusted.

While we're on the subject, if you are chasing the ultimate low action, be prepared and have a spare nut or a shim handy. The difference between buzzing and not is sometimes only one stroke of the file (especially if your nut files are new).

Slot width
The width of the slot should be slightly larger than the diameter of the string and the string should move through it easily without binding. If you need to make the string slightly wider than the file to accomodate the string without binding, it's possible to rock the file from side to side as you cut to widen it slightly.

Once you have filed to the correct depth, remove the gauges, reposition the first string and tune the string.

Repeat this for the second and third strings.To adjust the height of the fourth through sixth string, insert the gages from the bass side of the fingerboard and repeat the same process.

Raising the nut string height
If you found that the string height at the nut was too low, or you cut the string slots too deep, the string height at the nut can be increased easily.

One common and easy way of increasing the string height at the nut is to place a shim of wood veneer beneath the nut. To do this, remove the nut, cut a shim to the same width as the nut using a blade, glue the shim to the bottom of the nut and reglue the nut to the neck. Once the nut has been shimmed sufficiently, the string height can then be lowered, if necessary, using the same procedure as described above.