All modern guitars have truss rods inlayed in their necks. Truss rods were developed over 60 years ago to help reinforce guitar necks and keep them straight. Over time due to environmental conditions, guitar necks can warp or bow. The truss rod helps correct this natural tendency. Basically it's a rod that is inlayed down the center of the guitar neck. Basically it's a rod that is inlayed down the center of the guitar neck. When pressure is applied to the truss rod, tightening the truss rod, the truss rod bends inside the neck forcing the neck to bend with it. Loosening or tightening the truss rod will bend the neck in the proper direction to keep it straight. Generally speaking, the straighter the neck the more comfortable the guitar is to play.
Truss rods come in all different shapes and sizes, so do their access points. They have all different types of ends. The most common ends can be adjusted with screwdrivers, allen wrenches, and nut drivers. Some truss rods can be access through the truss where the fretboard ends. Others can be accessed at the bottom of the neck where the neck and body meet. Still other odd truss rods must be accessed through a back panel on the back of the guitar.
That is why there is no all-purpose truss rod adjustment tool. You will need a kit of truss rod tools if you plan on adjust many different guitars. The pocket truss rod wrench is used to adjust truss rods with hex shaped nuts that are accessible from the headstock and neck bottom. A pocket truss rod wrench is basically a nut drivers bent in a 90-dregree angle. The reason for this bend is to allow access to some guitars with odd access points. Some Les Paul guitars and other Gibson acoustic guitars' truss rod access point are inaccessible with a traditional nut driver. The angle of the headstock and the access hole make it nearly impossible to get in there with a straight nut driver. Even if you can get one in to adjust the truss rod, there is not even leverage to tighten the truss rod with a straight driver. You need a 90-degree angle wrench in order to tighten it. The pocket truss rod wrench also works well for guitars with a hex shaped nut truss rod end that have limited truss rod access in the bottom of the neck.
Pocket truss rod wrenches are great not only because they are among some of the only tools that work on guitar like a Gibson Gospel, but also they are really portable. Musicians can easily place them in the compartment inside their guitar cases and bust them out at a gig if need be. A truss rod pocket wrench set should be in your truss rod adjustment tool kit. You probably will need three different sizes of pocket wrenches: 1/4", 9/32", 5/16" nut sizes. These are great tools. I recommend getting at least one if not a full set. If you would like more information about how to adjust your guitar truss rod, please see my how to tighten and loosen your guitar truss rod article.