The alignment of the neck to instrument body is critical, and sometimes the fit isn't quite perfect at first. The body and neck are placed on a perfectly flat work surface, and coarse grit sandpaper with a sticky backing is attached to the body where the neck attaches. A pencil is used to shade the entire heel of the neck. The body is held down firmly, the neck placed against the body and flat to the work surface, and the neck end is slowly pulled across the sandpaper. This process must be repeated thousands of times, with the neck held perfectly square to the body and work surface. When all of the pencil shading is gone, the fit is perfect. You can see I still have a long way to go this morning.