Doublures

The definition of a doublure from the Webster dictionary is the lining of a book cover; especially an ornamental lining (as of tooled leather, painted vellum, or rich brocade).
There are many types of doublures, the photo above shows a sunken suede doublure, a contrasting colour edge to edge doublure and also a plaquette showing the process of how to prepare the surface for an edge to edge doublure (from left to right).
A sunken doublure is created by sticking a frame on the board prior to covering. This has to be planned in advance as the shoulder has to accommodate for this thickness. The turn ins are wider to cover the frame and you would cover the forth side of the frame the same way as you would do a leather hinge. The thickness of the material that you use for the frame would be the same thickness as what you would have for the inset.

With a same colour edge to edge doublure, the leather is pared very thin and feathered along the edges so that when it is stuck along the edge of the turn ins the feathered edge blends in with the turn in leather to create a seamless effect.

Contrasting colour doublures has to be pared along the edges really carefully as you want to keep the crisp line of the edge.