Page "Guitar" Paragraph 72

The truss rod is a metal rod that runs along the inside of the neck.

It is used to correct changes to the neck's curvature caused by the neck timbers aging, changes in humidity or to compensate for changes in the tension of strings.

The tension of the rod and neck assembly is adjusted by a hex nut or an allen-key bolt on the rod, usually located either at the headstock, sometimes under a cover, or just inside the body of the guitar underneath the fretboard and accessible through the sound hole.

Some truss rods can only be accessed by removing the neck.

The truss rod counteracts the immense amount of tension the strings place on the neck, bringing the neck back to a straighter position.

Turning the truss rod clockwise tightens it, counteracting the tension of the strings and straightening the neck or creating a backward bow.

Turning the truss rod counter-clockwise loosens it, allowing string tension to act on the neck and creating a forward bow.

Adjusting the truss rod affects the intonation of a guitar as well as the height of the strings from the fingerboard, called the action.

Some truss rod systems, called " double action " truss systems, tighten both ways, allowing the neck to be pushed both forward and backward ( standard truss rods can only be released to a point beyond which the neck is no longer compressed and pulled backward ).