A guitar’s truss rod is a steel rod that runs inside the neck; one end of the rod has a bolt that can be used to adjust its tension. You can adjust this part of the rod through the soundhole or the peghead. The other end of the rod is anchored to give the adjusting end something to tighten against.
Your guitar’s truss rod may need to be adjusted if you have a change in string gauges or even if the climate changes. Also, when buying a new acoustic guitar, it comes built according to the factory’s settings make which means it may not sound the way you want. One way to stylize your guitar to your preference is by adjusting your truss rod. You may also need to tweak your truss rod if your guitar’s action has stiffened over time, if you have too much backbow, or if you have too little upbow.
Adjusting the nut on your guitar’s truss rod will increase or decrease pressure on the rod and neck. A controlled upbow, also known as a relief, can be achieved by loosening the nut which brings the neck into a relaxed upbow position. To remove upbow, tighten the nut which will move the neck away from the string pull.
There are two styles of adjustable truss rods: a single action “one way” and a double action “two way”. A single action truss rod counteracts the upwards pull of the strings by bending the neck downwards. One-way truss rods are not so commonly used anymore. Before the convenience and flexibility of the double-action truss rod which became popular in the 1980s, musicians faced a major nuisance in the function of the one-way truss because if the neck warps away from the string pull, there is no way to reverse it. No amount of loosening the rod will pull the guitar neck straight because the rod only works AGAINST the pull of the strings.
There are two primary signs that your guitar’s truss rod needs adjusting. First, there is a change in the action caused by the height of strings over the frets. For example, the strings will get higher as the neck upbows from the string pull. Strings that are too high or too low need adjusting. Second, if the neck is too straight or backbowed, and you hear the strings buzz on the frets.
Hope this helps!