Double action trussrods of various lenghts are available. These allow the adjustment of fingerboard bows both convex and concave. They a simple to install as the bottom of the channel does not need rounding to make it deeper at the middle. There are also single action trussrods which do not allow the correction of back bows. They can be altered if necessary by shortening the length and bending the last inch or so over. All use a 4 mm Allen key for adjustment. Some are the narrower mini trussrods.