A truss rod is inserted into a guitar to compensate for the bend on the neck caused by the tension of the strings, usually on steel string guitars. There are some makers of classical guitars, that insert truss rods to allow more control of the neck relief and action
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 fret board 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 (approx. 0.175mm) 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.
Truss rods are frequently made out of steel, though graphite and other materials are sometimes used.
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).