Smyth Sewing: The Thread That Runs Through the Fold

A sewn book is held by thread that passes through the fold of every section. The machine that does it is a Smyth sewer, and it is the reason a hardcover can be opened flat after eighty years.

What the machine does

Signatures arrive folded, gathered in order, and lie in a stack in the hopper. The sewer opens each signature at the middle, pushes a row of needles through the fold from the inside, and passes a shuttle or a hook to catch the thread on the outside.

Each stitch runs through the fold, not through the side of the section. That is the difference between a sewn book and a side-stitched one. The thread sits in the paper's own hinge, so the pages turn without fighting a line of holes punched beside the spine.

Stations and stitches

The number of sewing stations sets the stitch spacing. A short, light book may run four stations. A thick art book may run eight or ten, and the spacing between them is measured in millimetres, not inches.

Thread size is chosen against the paper. A 3/0 cotton thread suits a 90 gsm text sheet. A heavier thread on light paper pulls the fold and shows a line of dimples down the spine. A thread too thin for a heavy signature breaks under its own load when the book is opened.

The kettle stitch

At each end of the signature the sewer makes a kettle stitch, linking the new section to the one below. That link is what makes the book a single block instead of a pile of loose sections.

The kettle stitch is also where a sewing fault shows first. If the link is loose, the sections fan apart at the head and tail and the spine gapes when the book is opened. If it is too tight, the thread cuts into the fold and the last section of each gathering tears at the spine.

Tapes and cords

Heavy books are sewn on tapes, wide fabric strips that run across the spine and are later glued into the case. The tapes carry the weight of the block and stop the sewing from stretching when a large page is turned.

A book sewn without tapes relies on the thread and the glue line alone. That is fine for a 200 page novel and wrong for a 700 page catalogue printed on coated stock.

Spine swell

Thread and the folds themselves take up space. A sewn block is thicker at the spine than at the fore-edge, and the difference is called swell. The case, the cover boards and the hinge must allow for it.

A case made to the trimmed page size without a swell allowance will not close over the block. The spine panel bows out and the boards sit proud at the fore-edge.

What goes wrong

Needles set too far apart leave a signature held at four points with a long unsupported fold between them. The book opens with a crack at each stitch and the fold splits after a few readings.

A blunt needle tears the fold rather than parting the fibres, and the tear runs into the page.

Thread tension that varies across the row tilts the block. The spine leans, the head is out of square, and the case has to be built to a block that is no longer rectangular.

Signatures sewn in the wrong order are almost impossible to fix after the glue line. The book has to be cut out of its case and resewn, which costs more than the original binding.

Sewn or glued

Perfect binding glues the trimmed edges of the sections into a cover. It is fast, cheap, and a paperback opens with a stiff spine that has to be broken in.

Sewing costs more per book and takes longer. It gives a block that opens flat, holds a heavy coated page, and survives decades of use. For a reference book, a Bible, or a case-bound edition sold as a keeper, the thread is the part that lasts.

What to ask a binder

Whether the book is Smyth sewn or side sewn, how many stations per signature, what thread and what size, whether it is sewn on tapes, and what swell allowance the case was built to. Then ask for a bound sample and open it at the middle. The spine should form a smooth curve, the stitches should stay buried in the fold, and no section should pull away from its neighbour.