Q&A: Mortise and Tenon on curved top rail

How would you go about doing the M&T joints of the back slats on a bench back with a curved top rail, like the one shown below?



Start out with a normal tenon that starts at the longest exposed end of the back slat. Lay the slat against the top rail and trace the angle needed at the joint. Then pare away excess slat material to allow the top rail to slide into position. The new tenon will be longer on one side to follow the curve of the top rail.