Truss rod

The truss rod is part of a guitar or other fretted, stringed-instruments that stabilizes the lengthwise forward curvature (also called relief), of the neck. Usually it is a steel bar or rod that runs inside the neck, beneath the fingerboard. Some are non-adjustable, but most modern truss rods have a nut at one or both ends that adjust its tension. The first truss rod patent was applied for by Thaddeus McHugh, an employee of the Gibson company, in 1921, though the idea of a "truss rod" appears in patents as early as 1908.

A guitar neck made of wood is prone to bending due to mainly two factors: 1.) atmospheric changes, and 2.) the pull created by changing to a different gauge of guitar strings. A truss rod keeps the neck straight by countering the pull of the strings and natural tendencies in the wood.

When the truss rod is loosened, the neck bends slightly in response to the tension of the strings. Similarly, when tightened the truss rod straightens the neck by resisting string tension.

Guitar technicians usually adjust a guitar neck to have a slight relief (forward bend) to achieve reasonably low action in high fretboard positions, while letting strings ring clearly in low positions. A lower action in the high fret positions also facilitates more accurate intonation 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 can make a neck feel floppy, slow and lifeless—while too little can make the strings buzz on the frets. Relief is typically measured as the distance between the string and the 7th fret while holding down the first and last fret. The amount of relief many guitar manufacturers prefer 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 builders to make instrument necks from less rigid materials, such as cheaper grade of wood, or man-made composites. Without a truss rod, many of these materials would be unable to properly handle string tension at normal neck dimensions. The neck can also be made thinner, which may improve playability. In fact, the 1923 patent touts the possibility of using cheaper materials as an advantage of the truss rod. Before truss rods, builders had to make the neck out of a very rigid wood, and achieve relief by laboriously planing the fingerboard.

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