The shoulder is a ridge on the inside joint of the book where it joins the cover board. When making a tacket, you punch a hole though the shoulder and thread a piece of linen thread through it.
Two holes are then punched diagonally though the cover board. The two ends of the linen thread are then threaded through these holes and a knot is tied.
By creating a shallow trench in the board, the knot and thread are embedded and glued into the cover, which is then not very visible.A piece of thin Japanese Moriki paper is then attached over the break in the joint.
Finally, another piece of Japanese Moriki paper is attached over the outside break in the joint. Several coats of a leather consolidant make the repair quite presentable as well as usable.