This is OSLO’s standard triple comb corner joint used on every window sash and all Euro door sashes, from the smallest to the largest in 2 1/4" thickness. Most common joinery in the industry is the single mortise and tenon which remains the weakest part of the sash structure. OSLO's triple comb joint offers over three times the gluing surface of mortise and tenon, and the corner becomes the strongest part of the sash.