Abstract: A unique modified shiplap joint with a transverse spline extending into the respective edge of each piece of siding forming the joint. The spline ensures that the two adjacent pieces of siding are properly and adequately connected to each other, and, through the nail in the underlap portion of the joint, to the wall. The spline may be located in four possible positions: (1) attached to the back side of the underlap section of the joint (i.e., closest to the wall); (2) through the center of the underlap section of the joint; (3) directly attached to the front face of the underlap section of the joint; and (4) through the overlap section of the joint (i.e., closest to the outside face). The spline may be installed on one of the pieces of siding during manufacturing or at the factory, or alternatively, at the job site.