The following is an excerpt from How to Make Your Electric Guitar Play Great, second edition, by Dan Erlewine, published by Backbeat Books/Hal Leonard Performing Arts Publishing Group. Re-printed with permission.
Adjustments are what setups are all about, and the first adjustment to make is the truss rod. Almost all electric guitars have adjustable truss rods, and few setups would be complete without tweaking the truss rod.
Occasionally a guitar neck is perfect as is, and doesn't require adjustment. If you have one of those, don't mess with a good thing. This chapter will show you how to recognize a neck that's perfect, and how to adjust a neck that's up-bowed or back-bowed. Our goal is a state of controlled straightness, with a very slight curvature called "relief."
Up-bow refers to a fretboard that curves in the direction of the string pull, creating a valley under the strings. This makes for high, stiff action.
Back-bow refers to a fretboard with a hump in the center, occurring when a truss rod is so tight that it bows the neck away from the string pull. Back-bow makes a guitar completely unplayable because the strings buzz against the humped frets.
Relief is a controlled up-bow, deliberately adjusted into a straight neck to create string-to-fret clearance that allows for the strings to vibrate without buzzing. Not every guitar benefits from relief, and not everyone likes it. The choice between relief or a straight neck is up to the player. But such a great majority of setups require relief that you can consider it a standard.
Truss rod control of the fretboard's straightness goes hand in hand with setting the string height at the bridge and the nut. These adjustments together produce the playing action, so a professional will simultaneously adjust a truss rod while raising or lowering the bridge and measuring string height at the nut. This process involves watching, measuring, and adjusting the neck, then measuring, watching and adjusting some more. It's a dance involving all of this at once, so you'll need to refer to the nut and bridge chapters as you work with the truss rod information presented here.
Truss rod adjustment tools
You need very simple tools to adjust truss rods. Many guitars adjust at the peghead with a socket driver or an Allen wrench; vintage Fenders and their many clones adjust at the body end of the neck with a flat or Phillips head screwdriver (some imports use an Allen wrench). Usually, imports require metric tool sizes and American-made guitars require fractional and decimal tool sizes. Depending on the guitar brand, the tool you'll need will be one of these:
Flat blade screwdriver: 5/16"
Phillips head screwdriver: No. 2
Socket driver: 1/4", 5/16", 9/32", 7mm, or or 8mm (9/32" will substitute for 7mm)
Allen wrench: 1/8", 9/64", 3/16", 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 5mm, 7mm, or 8mm.
Truss rod types and how they work
The one-way, or single-action, rod is a very common style of adjustable truss rod. It's just a round steel rod with threaded ends. It lays in a very shallow curve, with one end anchored in the wood of the neck. The other end has the adjustment nut. This rod is designed for one action: to pull back against the tension of the strings, straightening out up-bow in the neck. Tightening the rod pulls it back away from the string tension, while loosening it allows it to be pulled back up by the strings.
There are other designs, such as a U-shaped metal channel instead of a round rod, but they're performing the same function. These typical designs are one-way, or single action, truss rods. They apply tension in one direction only.
Some truss rods are two-way designs. When tightened, they exert pressure against the strings like a one-way rod, but if turned the other direction, they will push the neck forward rather than relying solely on string tension.
Some necks are not adjustable at all, with a truss rod that is simply a stiffener embedded in the neck (rare these days). And some older inexpensive guitars such as Kays, Harmonys, and early imports have adjustable rods that work so poorly they may as well be considered non-adjustable. These are rare too, and it's a pretty safe bet that your guitar's truss rod will do the job.
Some adjusting nuts screw onto the threaded end of the truss rod (vintage Gibson, Guild, Gretsch, and Fender models), and these can be removed for cleaning and lubrication. On other designs, such as PRS guitars, the adjusting nut is welded to the rod and can't be removed.