What does a truss rod do?
A truss rod is needed on a wood necked bass to keep it stable from warping due to humidity, string tension, other weather related issues. Truss rods are usually set to be adjusted: *clockwise = make gap between strings and neck less (straighter neck) *counterclockwise = make spacing between strings and neck greater (more bowed neck). Here is some good advice on truss rod adjustment. There are also dual truss rod basses for even finer adjustments.
