These are images from the first sewn boards binding I completed a few years ago. Gary Frost developed this structure and it is incredibly beautiful both in the way it functions and its clean aesthetic. The boards are made from a stiffer, 20 pt card that are folded and sewn on just like another signature in the book (this is what you see as tan in the above photos). Once the textblock is completely sewn, you can insert another stiff card in the folio (this is the black area inserted between the tan sections). These three layers act as the boards for the book, which is then covered with three separate pieces of either paper/cloth/leather.
I believe the original structure was developed as a conservation binding, to re-sew and preserve older books. But Gary Frost also mentions that it is a good structure for edition binding. You can read more about this binding at Gary Frost's website.