a lengthwise miter joint and boards for forming it have mating effective v - tongue and - groove portions in their miter faces for positive registered alignment of the joint while it is being permanently fastened as by gluing , nailing , etc . the tongue and groove portions are centered widthwise of their respective miter faces so that the same boards may be selectively assembled either in angular or aligned relation to each other . the effective included angles of the tongue and groove provide registered angular assembly of the joint with a single rectilinear motion of one board perpendicular to the flat sides of that board . the effective v - tongues and - grooves characteristically provide relatively strong shapes to both boards in a joint .