The truss rod is a metal rod that runs along the inside of the neck. Its tension is adjusted by a hex nut or an allen-key bolt usually located either at the headstock (sometimes under a cover) or just inside the body of the guitar, underneath the fretboard (accessible through the sound hole). Some truss rods can only be accessed by removing the neck, forcing the luthier to replace it after every adjustment to check its accuracy. The truss rod counteracts the immense amount of tension the strings place on the neck, bringing the neck back to a straighter position. The truss rod can be adjusted to compensate for changes in the neck wood due to changes in humidity or to compensate for changes in the tension of strings. Tightening the rod will curve the neck back and loosening it will return it forward. Adjusting the truss rod affects the intonation of a guitar as well as affecting the action (the height of the strings from the fingerboard). Some truss rod systems, called "double action" truss systems, will tighten both ways, allowing the neck to be pushed both forward and backward (most truss rods can only be loosened so much, beyond which the bolt will just come loose and the neck will no longer be pulled backward). Most classical guitars do not have truss rods, as the nylon strings do not put enough tension on the neck for one to be needed.