three-piece cases

The three-piece case consists of two boards joined by a a spine-piece of thick paper, cartonnage or thin millboard, creating a primary cover which may then have a secondary cover, usually coloured and often decorated, adhered over it. Three-piece cases were made in two different basic types, cut flush and turned-in.
The German term for the three-piece case, 'gebrochener Rücken', meaning literally 'broken back', is presumably a reference to splitting a one-piece case into two sides with a connecting spine-piece. This meant that it was possible to have a thinner flexible spine-piece that allowed the book to open whilst having a rigid board on each side to support and protect the bookblock, a dual function that was not possible with the one-piece case.