Twin comb joints
Sash windows for centuries have been constructed using the tenon and mortice joint, a good joint suited to the design and weight of a single glazed sash window.
Double glazed sash windows however, being much thicker and twice the weight, place alot of stress on the corner joints. To address this structural stress, we use, as standard, a twin comb joint.
Whilst similar in many ways to a tenon and mortice joint, the big difference is a much greater surface area for gluing. It is this factor that makes the twin comb joint much more suitable for a double glazed window, as ulimately a greater gluing area gives greater strength to a joint.