This is a compound angle joint using the Domino. On the top piece you can see the center line with the angle line next to it. The vertical member is beveled/mitered 2.5º.
To mortice the vertical member I set the fence to 2.5º, place it against the member and plunged. The horizontal member is flat on both planes across the bottom. To plunge the member I left the fence at 2.5º, aligned the center line of the Domino on the angled mark. To stabilize the Domino I placed a shim between the fence and member. This method create mortices that were aligned and on plane with each other. Edges lined up perfectly also. Did the same on the bottom of the vertical members also.
Tom