Shiplap boards are milled on both edges, and often on the back to create a tight fitting side to side boards with half of the thickness of one board overlapping half the thickness of the board next to it; the overlap creates a more moisture resistant joint than a board to board joint, and if there is seasonal board shrinkage the lap will shield the substrate from view; shiplap can also be milled so that the joint has a channel between the two boards, often called channel lap, creating the feeling of depth.