This cutaway view of a headstock end shows a single-action truss rod and its adjustment nut.
Do you know what a guitar’s truss rod does or how it works? When I first started playing guitar, the truss rod was a riddle. Sometimes erroneously called the “trust rod,” or even the somewhat provocative “thrust rod,” it remained a cipher waiting to be decoded for me. Did I dare to move it to unlock its purpose? Of course there is plenty of information about adjusting your guitar’s neck nowadays, but what are the origins and variations of this device?
What is it for? Most simply put, when you tighten the truss rod, it bends the neck backwards against the pull of the strings. There are forces at work on your guitar’s playing surface—that is, the neck and fretboard—and the force of the strings wound to pitch is bowing the neck forward with over 100 pounds of tension. Even if compensated for by the builder, a condition known as “creep” changes the amount of forward bow over time.
Additionally, humidity in the air (or lack of it) can swell or shrink the fretboard, causing it to bend the entire neck forward or back. These conditions must be controlled accurately in order to allow a guitar to fret precisely. The truss rod is the pillar of strength and the methodical adjuster that allows your guitar to function.
Before the advent of steel, stringed instruments (including the guitar) used lightweight strings of animal origin, which exerted relatively low amounts of tension upon the necks. So, a wooden neck alone was sufficiently strong and a builder could anticipate the amount of bow, and build in preemptive compensation.
As metal strings gained popularity towards the end of the 19th century, builders became concerned about the ability of their instruments to withstand the added force of steel. By the early 1900s, Gibson’s multi-stringed Style “U” harp guitar incorporated an external metal rod equipped with a turnbuckle adjustment to counteract the huge tug of multiple steel strings. This undoubtedly led to Gibson seeking a remedy for their 6-string guitars as well, which resulted in the first truss-rod patent that was filed on April 5, 1921. The document, U.S. Patent No. 1,446,758, shows a single steel rod of approximately 3/16" diameter that could be shortened via a nut located at the headstock end.
Thus was born the ancestral patriarch of the rod configuration used by almost every builder today.
So how does it work? A guitar neck is divided by a “neutral plane” as master builder Ken Parker calls it—a physical location between the face of the fretboard and the back of the neck where the neck neither stretches or compresses when bent. “Think of a diving board,” says Parker. “When you stand on the end, it bends. The material at the top is stretching but the material at the bottom is compressing. There is a point in-between, he continues, where neither is occurring.”