Wood sidings are milled in various patterns for both horizontal and vertical application. Wood siding is usually, but not always, applied over solid sheathing and a layer of building paper. Horizontal patterns are nailed through sheathing into studs; vertical patterns generally are nailed to horizontal nailing or furring strips. Horizontal sidings are always applied so that each board overlaps the one below it; in some cases, the edges are milled to interlock, as they are with all vertical patterns.