Y corner-post joint with open slot mortise and tenon

The T corner-post joint with open slot mortise and tenon is a Y-joint where a vertical post is provided with an open slot into which two horizontally running members engage from opposite sides with through-tenons. The connection is created by the meeting of three members at a node, with the corner post receiving and connecting the two sills or plates. This three-way connection enables the right-angled arrangement of members without additional fasteners.
This joint belongs to the tenon joint family and is also referred to as sammai-gumi or open slot mortise. It shows relationship to the wanagi-komi and other multiple tenon joints. Unlike simple corner joints, this construction enables the meeting of two horizontal members with a vertical post at a corner, creating a stable three-dimensional frame structure.
The geometry consists of a vertical corner post that is provided with an open slot across its entire width. This slot divides the post into two parallel cheeks. Into this slot engage the through-tenons of the horizontal members from two perpendicular sides. The tenons are approximately one-third of the member thickness and penetrate the post completely. The shoulders of the tenons rest against the cheeks of the post and transmit compressive forces. The open construction enables assembly by lateral insertion of the members.
The T corner-post joint with through-tenon offers a robust corner connection for frame constructions in timber building. The open slot form enables simple assembly without complex jointing techniques. The corner post remains continuous in its length and is not weakened by mortises. The connection is form-fitting and requires no additional fasteners. The through-tenons enable visual quality control of the connection. However, the corner post is significantly weakened by the continuous slot. The open construction makes the connection susceptible to lateral twisting. With improper fabrication, the connection can develop play and lose strength.
Traditionally, this joint is used in groundsill construction for corner posts that receive two perpendicular sills. In timber-frame construction it is used for studs that connect plates in two directions. In wall and roof structures it is employed for corner connections where a vertical post must receive horizontal members from two directions. The joint is particularly suitable for right-angled frame structures where assemblability and continuous post guidance are important.
Fabrication is carried out by precisely marking the open slot in the corner post, accurately sawing the parallel slot walls, and cleanly chiseling out the intermediate space. On the horizontal members, the through-tenons with their shoulders are marked and the protruding material is sawed off. Fitting accuracy is crucial for the strength of the connection. The members are inserted from the sides into the slot, ensuring that the tenon ends remain visible on the opposite side. For visible applications, careful surface finishing of the protruding tenon ends is required.