Interconnecting splines are typical of panel-to-panel attachment. Splines are 4-inch wide strips of cement board running fully the length (or height) of panels, parallel to, and at both the interior and exterior faces. A 2 in x 4-inch dimensional lumber is used as an interior spline when sheetrock is the skin of the panel. At panel joints, the splines are shared by adjacent panels with each panel edge accepting half of the common splines. Panels are manufactured with the expanded polystyrene (EPS) recessed by the thickness of the spline. Splines are typically the same thickness as the panel skins to which they attach. The thickness of the panels is determined before construction according to loads and thermal needs.