The traditional reason given for using a joint groove is to allow the covering material to bend backwards in a more gentle manor, so that the board swings opens more widely without bunching up on itself. This was part of the reason the joint groove made a comeback in the early years of the twentieth century on leather bound books, along with the split-board structure, as a way to use thicker leather in order to make books more durable.
This, however, is only part of the story. Adhered-boards structures are very common in early nineteenth century American leather, paper and cloth bindings. Instead of laced boards, the slips are pasted to the outermost endleaf. Then the boards are adhered to this for about an inch or so. After this is dry, the unpasted area of the endleaf is torn off, and the boards can be trimmed to size.
As you can see in these images, the joint groove significantly affects the opening, even without any covering material. Note that it is the same book in both images, with the front board placed tight, and the back board with a joint groove.The difference surprised me. There is always something new to learn: this is what keeps me interested in the history and practice of bookbinding.
Peachey Conservation 
Thanks for the post Jeff!
Yes, there’s always something to learn, and it is certainly surprising how the structural changes modify the whole thing, even when it’s only some paste, threads and paper…
We love bookbinding!
🙂
Iam confused exactly how to make the French groove. Do you apply paste deep into the groove prior to applying the leather and force the leather into the grove with a rod or folder? If not, where do you paste to hold the groove in place?–on the cover board edges along the spine?
In this binding style, the boards are adhered to an endleaf before it is are covered. The groove results from spacing the board slightly away from the edge of the shoulder. Then when the book is covered, as you mention, the covering material is worked into the groove, often with a folder. Brass edge boards can be used to keep the groove defined when the book is drying in a press.