In reply to Mary E Yordy.

Thanks, Mary! Yes, you bring up an important point. I think the sewing tension plays an important role in forming the shoulder of the textblock. If there is a tight connection between the board and the first (or last) section, the backing naturally forms when the book is closed. And this seems to be the advantage of sewing the textblock in two halves: it's easier to get even tension and form matching shoulders at the front and back of the textblock that way. It's probably possible to achieve by sewing from one board, through the textblock, and into the other - but I think it would be tough to lace that second board on as tightly.