A truss rod is a rigid shank that runs under the fret board and runs the length of a guitar neck. It can be made of wood or graphite, but steel is the most common material. This inner rod aids in the counterbalancing of string tension on the neck, allowing the guitar’s neck to remain relatively flat. For added stability, some guitars, such as Rickenbackers, have dual truss rods.
Truss rods can be adjusted to prevent neck bow (also known as “relief”) from developing over time. The guitar neck can be affected by seasonal changes, humidity, and changing between heavier and lighter gauge strings. The truss can only be adjusted from one location, which varies between guitar models. The adjustment head is usually hidden behind a small plate, but it is visible on some guitars.
The rod adjuster on electric guitars with bolt-on necks is usually hidden under the pickguard plate at the base of the neck. Older Fenders® and Fender-style copies, many Yamaha® guitars, and other models, including vintage reissues, all have this setup.
The adjuster on newer Fender guitars is located at the base of the headstock, just above the nut. The nut is the small piece that guides the strings off the headstock, and the headstock is the portion of the guitar where the tuners or keys are located. On these models, the truss rod adjuster is exposed, but recessed (as always) in a concave hollow.
The truss rod adjuster is located in the same location as on newer Fenders on set-neck guitars, or guitars whose necks are glued on with adhesive rather than bolted on. The adjuster is covered by a small plate fastened with screws in the set-neck models, however. This category includes Gibson® and Epiphone® guitars.
On steel-stringed acoustic guitars, the adjuster can be placed in one of two places. It will either be accessible through the sound hole on the inside heel of the neck, or at the base of the headstock, as previously stated. The truss rod will be hidden behind a plate in the former case, but it will be visible in the latter if it is located in the sound hole.
Truss rods are used on electric and steel string acoustic guitars, but they are not required on classical guitars. These guitars use nylon strings, which do not exert the same amount of tension on the necks as steel-stringed guitars, obviating the need for reinforcement.
While truss rod adjustment isn’t difficult, getting it wrong can be a costly lesson and potentially ruin your guitar. If you don’t feel comfortable adjusting the neck of your guitar on your own, take it to a shop. Adjustment by a professional is a low-cost service.