The truss rod is part of a guitar or other fretted, stringed-instruments used to stabilize and adjust the lengthwise forward curvature (also called relief), of the neck. Usually it is a steel rod that runs inside the neck and has a bolt that can be used to adjust its tension. The first truss rod patent was applied for by Thaddeus McHugh, an employee of the Gibson company, in 1921  , although the idea of "truss rod" can be encountered in patents as early as 1908. 
A guitar neck made of wood is prone to bending due to mainly 2 factors, atmospheric changes and the pull created by changing to a different gauge of guitar strings. The Truss rod is designed to counter the pull of the strings and natural tendencies in the wood in order to keep the neck straight.
When the truss rod is loosened, it allows the neck to bend slightly in response to the tension of the strings. Similarly, when tightened the truss rod straightens the neck by resisting the tension of the strings.
It is desirable for a guitar neck to have a slight relief in order that reasonably low action be achieved in the high fretboard positions, while at the same time, the strings ring clearly in the low positions. A lower action in the high fret positions also allows for more accurate intonation to be achieved with less compensation at the bridge.
Relief achieved through the truss rod combines with the height of the bridge to affect the playability of the instrument. The two should be adjusted in tandem. Too much relief contributes to a neck that feels floppy, slow and lifeless, while too little will allow the strings to buzz on the frets. Relief can be measured as the distance between the string and the 7th fret while holding down the first and last fret. The amount of relief preferred by many guitar manufacturers for an electric guitar is about .007 inches at the 7th fret.
Truss rods are required for instruments with steel (high tension) strings. Without a truss rod, the guitar's wooden neck would gradually warp (i.e. bend) beyond repair due to applied high tension. Such devices are not normally needed on instruments with lower tension strings, such as the classical guitar which uses nylon (previously catgut) strings.
Truss rods also allow the instrument neck to be made from less rigid materials, such as cheaper grade of wood, or man-made composites, without which the neck would not be able to properly handle the string tension. The neck can also be made thinner, which can improve playability. In fact, the possibility of selecting cheaper materials is specifically touted in the 1923 patent as an advantage of the truss rod. Prior to the introduction of truss rods, the neck would have been made of a very rigid wood, and relief was achieved by planing the fingerboard: more expensive material, and more demanding construction technique.
The truss rod is not for adjusting the intonation or the action (height of the strings above the fingerboard) although adjusting it can cause an instrument to become more easily playable.
Construction and action
The truss rod can be adjusted to compensate for expansion or contraction in the neck wood due to changes in humidity or temperature, or to compensate for changes in the tension of the strings (the thicker the guitar string, the higher its tension when tuned to correct pitch).
Usually, the truss rod of a brand-new instrument is adjusted by the manufacturer before sale. Normally, turning the truss rod's adjustment bolt clockwise tightens it, counteracting the tension of the strings and straightening the neck or creating a backward bow. Turning the bolt counter-clockwise loosens it, allowing string tension to act on the neck and creating a forward bow (higher string action).
Some guitars (notably Rickenbackers) come with dual truss rods that are more stable and not affected by seasonal climate changes. They are regarded as being more difficult to adjust.
Location and adjustment
- On older Fender-style electric guitars with bolt-on necks (and vintage re-issues) — on the heel of the neck. Adjustment of such truss rods can be done by a Phillips screwdriver and requires prior removal of the guitar's pickguard or neck.
- On newer Fender-style electric guitars — behind the nut, uncovered and can usually be adjusted by a 1/8" (3 mm) Allen wrench.
- On set-neck electrics — under a cover-plate behind the nut. Gibson & Epiphone guitars have their truss rod bolt covered with a signature bell-shaped plate. Most Gibson electrics have a 5/16” (8 mm) or a 1/4" (6 mm) hex adjustable truss rod nut that can be adjusted with a hex box spanner wrench.
- On acoustic guitars — inside the guitar body, accessible through the sound hole, or on the headstock. Martins use a 3/16" (5 mm) Allen wrench and Gibson uses the same as for the Gibson electrics above.
Modern designs also include adjustment from the side of the heel of a bolt-on neck. When looking from the body of the guitar to the head, counterclockwise adjustment decreases the truss rod tension (correct an underbow) and clockwise adjustment increases the truss rod tension (correct an overbow).
Installing a truss rod in a newly constructed guitar requires woodworking capabilities. Special tools are required including a router with a variety of bits and ability to work with metals. Completed truss rods can be purchased through suppliers or manufactured according to specifications given in literature.
Dual action truss rod
A dual action (also known as two way or double expanding) truss rod is a more modern design and it is currently being used by some luthiers (notably Warmoth) in lieu of the vintage single truss rod. The dual action rod is installed in a straight channel in the wood as opposed to the curved channel used by vintage rods. The two-way rod can warp the neck in either direction, either creating more relief or less.
A neck with a two way rod installed is often more stable and less influenced by climate changes, as well as being able to restraighten twisted necks, which can be a hefty repair for other guitars. However, some players believe the dual action truss system has an adverse effect on the tone of the instrument, due to the weight of the second rod and the additional wood removal required for installation. (Actually, not all two-way systems use a second rod)
- US patent 1446758, Thaddeus McHugh, "Neck for musical instruments", issued 1923-02-27 — first patent known on truss rod. Note that the diagram in McHugh's patent document shows an incorrect design for the truss rod channel. The channel is shallower in the middle, causing the truss rod to be installed with a backbow. Tightening the rod increases neck relief, acting in concert with string tension, rather than reduce relief in opposition to string tension.
- US patent 964660, George D. Laurian, "Stringed musical instrument", issued 1910-07-19 — a patent on instrument that is "designed like harp guitar", but it mentions "truss rod" concept "to prevent a head springing under the strain of strings".
- Truss Rod Adjustment article on the Guitar Repair Bench Luthier Website
Simple guide on how and why to adjust the truss rod created by Guitarbitz Guitar Shop