Abstract: A concealed joint construction system for installing and securing sheet material such as laminated plastic wall boards, plywood and the like for finishing off bathtub and/or shower walls, counter-tops and back-splashes, and the like. The joint construction system includes an extending channel attached to one wallboard via multiple teeth sunk into matching grooves cut out of the wallboards edge. An adjacent wallboard can be easily and quickly inserted into the extending channel and is secured by pressure flanges perforated from the wall of the channel. The joint construction system is easily installed, secure, and compact, and is completely invisible after installation.