Bookbinding - A style of spine construction where the outer material is attached directly to the spine of the book block.
Also known as a tight back this is the oldest style of spine construction. It is called "flexible" not because it makes the entire book more flexible, but rather because the outer covering of the spine must flex when the book is opened. It is most commonly used when sewing on exposed cords.
___________ ___________ \
__________ \ / __________ > <- pages
_________ \ | | / _________ /
==========/ /|\ \========== <- cover
Until the late Eighteenth Century, flexible binding was the only way to attach a cover to the spine of a book block. The difficulty of this technique - making a spine that combines flexibility with enough strength to withstand years of bending - kept bookbinding a specialist art for centuries.
An alternative spine treatment, the hollow back, arose in France at the end of the Eighteenth Century. Along with French grooves, hollow backs made bookbinding easier, and books cheaper, just when the rise of the middle classes increased demand for private libraries.
The years just before the turn of the Twentieth Century saw a return to flexible bindings in fine bookbinding. The Arts and Crafts binder T.J. Cobden-Sanderson was the leader of the movement back to this style, which was seen as more craftsman-like. It is still the mark of high-quality hand binding. A good flexible spine is regarded as the sign of a highly skilled binder.
Flexible bindings are commonly sewn on exposed cords rather than tapes or buried cords, and generally use tight joints at the covers. They are not appropriate for very thick books. A less common style, library binding, uses French grooves with the flexible spine.