In bookbinding, swell refers to the increased thickness of a textblock along its spine edge after sewing.
Human factors also influence swell, especially the force with which the bookbinder "bones down", or flattens, each section after sewing.
Prior to the 15th century, swell in textblocks was accommodated, if at all, by shaping the wooden boards that formed a book's covers.
The inner surface of each board would be lightly tapered along the spine edge, so that the removed wood created a space for the thicker paper along the sewn folds.
Too much swell, however, causes the spine to become over-round, and can create a propensity among the middle sections of the book to collapse inwards, falling toward the fore edge.