If you talk to a guitar enthusiast it may well be that they have heard of truss rods, and may even be capable of showing you where their truss rod is to be found, but to the average person or beginner, most people would be hard pressed to know whether their guitar even had a truss rod. So what is this feature, and what does it do? First of all, if we think about the problem that may be caused by not having a truss rod, then we can see where its role fits in.
A guitar has a long neck, the fret board, up which runt eh strings. It is this long board which is used by the secondary hand to press the strings down in an arrangement which will create the chord to be played. Typically most guitars are made of wood, and so this fret board is a long, slender wooden board. Not only that, but the fret board is under a great deal of pressure, since the strings are pulled very tightly indeed. In fact, on many guitars the strings are made from steel, and tightened quite considerably, putting the fret board under a great deal of pressure. Wood is a natural product, obviously, and so therefore prone to the same problems that all natural products suffer from, and that is aging and deterioration.
Certainly guitars, as with most wooden products, are treated carefully with sealants and varnishes to help nourish and protect the guitar for a good long life. However, with most wooden products the problem of slight warping may only lend character, for a guitar, and warping spells disaster, since the design and measurements of a guitar are very exact, to ensure that the notes are crisp, clear and accurate. Because of the length of the fret board, any warping or bending could cause the notes played to be altered quite noticeably, and so these would render the guitar useless. Not only that, but slight warping can lead to cracking, and with so much pressure on the fret board already from the strings, this would lead inevitably to breakage of the fret board completely. The truss rod, therefore, is a strong metal rod which runs up through the fret board from one end to the other, and can be tightened usually using an Allan key at the top.
This metal rod acts a little like a backbone for the fret board, meaning that aging, humidity and any other causes of natural deterioration will not result in warping or disfigurement of the fret board, helping to ensure that the notes still sound clear and accurate, and the life of the guitar is lengthened. Most classical guitars only use nylon strings, and so there is far less pressure on the fret board. For this reason, you may not always find truss rods included, but where you do find them, they can sometimes come with an extra feature which allows an alteration in the tone of the notes played to be heard. By adjusting the truss rod by turning it either clockwise or antic clockwise, the fret board can be adjusted to an angle either side of the normal centre. This can adjust the distance of the strings from the fret board, allowing a different playing method to be adopted, as well as altering the tone very slightly.
Those truss rods that allow movement in both directions are known as double truss systems. In some cases the truss rod can only be accessed by completely removing the headstock of the guitar, which is the part at the very top of the fret board, and so altering the style of the guitar is something done only very rarely.