This style of binding was in use around the 8th - 12th century Europe and was named after the Frankish dynasty. It is sewn with herringbone stitch on several double cords which are laced in the wooden boards. In my case the boards are oak but according to J.A. Szirmai, beech and poplar were also used depending on the location. As I didn’t have any nice monastic vellum manuscripts on hand, I used a large stack of recycled paper for the text block of this model.
In the pictures you can see I have pasted on the leather reinforcement tabs. I don’t have a picture of the headbands but they were sewn with simple link stitches around the two cords laced in the cover boards, with several tie downs through these tabs. The tabs were then cut to desired shape and sewn together with similar extensions in the covering leather. In original bindings chamois was used for the covering - and in some cases local deerskin by Charlemagne’s permission granted to the Abbey of St. Denis.
The tabs most likely existed to ease the handling of the manuscripts as books were commonly stored flat, in chests or stacked on each other. They do have some aesthetic qualities too: the tabs push the leather on the spine to a clear concave arch when the book is opened, and the stitching along the edge of the tab can look quite nice. Carolingian bindings also had clasps, which were leather strips ending in a metal D ring that closed on a small peg nailed on the edge of the cover. The leather strap was accommodated in a chiseled recess in the other cover board before pasting on the leather.
Carolingian bindings are typically quite plain, although some examples remain of French books with simple tooling. This, combined with the large size and the text block cut flush with the cover, gives the finished book an essence of a big rectangular lump. This is also one of the reasons why I didn’t finish my model. I made the first one with haste, and with a goatskin cover it ended up looking quite dull so it has been already repurposed as a beaten-up personal journal (not mine, though). This model serves as a good reference on the basics.
Nice reading: The Archaeology of Medieval Bookbinding by J.A. Szirmai Making the Medieval Book - Techniques of Production by Linda L. Browrigg