Paper is thin, but the thickness of a pile of sheets soon adds up to make a thick book. It is the same with thread. In a closed book there are many layers of thread, buried between the pages but adding bulk to the book's thickness. This extra is aptly named as the swell.
Avoiding swell, or utilising it to good effect, are alternatives available to the bookbinder, in addition to ignoring it.
Ignoring swell is only possible with thin threads and large sections. Modern polymer threads can be made very thin, and very strong. I notice in some books that the thread is doubled-up, with little noticeable swell.
The traditional round-backed book makes use of the swell, caused by the thicker linen threads. The extra thickness is amplified as the backs of the sections are gently hammered and beaten into a mushroom shape. The overhang of the mushroom is carefully measured to be equal to the thickness of the covering boards.
One use of swell is to exaggerate it. We do this, occasionally, when making an album in which many thick photographs or embellishments are expected. Thread as thick as string acts as a very good spacer, and to make every folio a single section will allow plenty of extra thickness to the spine.
