Right:A trouble-free way of shaping a rail and leg and maybe the stiles and rails of a frame.
Top right:It is virtually impossible to shape the underside of a rail in this way.The short grain at the end of the curve would be so fragile that it would crumble as you try to work it.
Though it still need careful working, the mitred shoulder makes this kind of curve a possibility.
Bottom left:An exploded view of the joint
Hints for making: