When making a dovetailed plane, 1/16" surplus metal is needed over the finished size to form the dovetails. This just shows 2 pieces of metal that I carry around to show people how to form the dovetails on a plane.
This shows a straight forward dovetail joint as in wood, the bottom socket shows the arrises filed away for the next stage.
This shows the next stage ready for peening to form the second dovetail.
The arrises have been filed away on the left and straight sides on the right.
When put together like this, the right hand side as in wood, the left hand side as in plane making to form the double dovetail. You can just see on the left hand side joint the amount you have got to peen to form the second dovetail. (the impossible joint)