The plough is cut in before shaping the curved thumbnail. It is scored with a marking gauge and then chiseled down to a consistent 1/2" depth. This is very important because when fitting the feather edge of the top of the arched panel it must not have any obstructions. Also, the carved feather edge of the arch must be perfectly uniform in thickness all around the curve to fit in the plough to have even dimensions, or, it will look bad, and we can't have that ! >