Truss Rod Ball-end Allen Wrenches
All modern electric and acoustic 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. 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. Ball end wrenches are basically allen wrenches or hex key wrenches that have the corners of the hex end rounded. The rounded hex end allows you to rotate the wrench from all different angles. Normal allen wrenches need to be completely straight on and lined up with truss rod in order to fit into the nut. Ball end wrenches can be at a 45-degree angle with the truss rod nut and still tighten and loosen it! This is very handy when you are tightening a truss rod in the headstock. Most of the time the access hole is angles, so you can't fit a straight wrench in the truss rod end. A ball end wrench will fit just fine.
Ball end wrenches are pretty standard tools. There is nothing guitar specific about them. They are just another handy tool to keep in your guitar repair bench. Obviously, you can use them for other repairs and even around your house. I would invest in a set of them because you need a few different sizes to adjust truss rods. It's probably cheaper to buy a hole set than just get a few individual wrenches; however, these are guitar repair tools that you can live without. You don't really need these to do anything specific. If you would like more information about how to adjust your guitar truss rod, please see my how to straighten your guitar neck with the truss rod article.