Before cutting the coulisse and fitting the heel, a piece of shell is cut into a rectangle and glued to a strip of ebony. Now it is ready to fit by filing an angle or dovetail on each side and gradually bringing it down until it slides in snugly against the heel. The coulisse or channel is slightly tapered which ensures a tight fit. It would not be good for the hair underneath to push the slide up. Then the other end is trimmed carefully until the ferrule can slide back on and fit tightly against the shell. But the ebony strip underneath continues on under the ferrule to prevent the slide from lifting.
This particular piece of nacre or shell is very beautiful but for some optical reason it’s difficult to record this in a photograph. When the pearl slide is fit the frog is nearly finished and only the under-slide where the frog fits on the stick remains to be done. Since the stick hasn’t been chosen I will put the frog aside for now and think further about the bow and the choice of wood.