More glue, more waiting around for glue to dry.
A muslin hinge called a super is glued to the spine edge of the text block.
After that dries, the flaps of the super are glued to the front & back boards, followed by the tapes, followed by the endpapers.
If all goes well, the book will open & close properly.
A "real" spine!
And now the part I love best of all—the way the book sounds when you flip through the pages. Grab some popcorn and enjoy the movie.