This is an ash box frame which has a dovetail oak key fitted on the face of the mitre, simply as a decorative effect. The dovetail is made using a Hoffmann router, usually it is used as a dedicated frame joining system, but from the back of the mitre and using a plastic key instead of a wooden one. With this frame the 10mm deep recess is cut, then the frame is glued and joined on an underpinner, then glue is put in the recess and the oak key is fitted. lastly the frame is strap clamped to give good tight corners all round.
