Some guitars have the neck glued to the body (Les Pauls), some are bolted on (Fenders), and in some the neck is part of one piece of wood that extends through the body (Jackson). The theory of the neck through the body design is that if the pickups, bridge, and the strings (from tuners to tailpiece) are mounted on the same piece of wood, sustain will be greatly enhanced. In practice, a well joined glue-on and a properly fitted bolt-on will sustain just as well.