The sewing thread is looped completely around the cords, instead of passing in front of them. This type of sewing may be done on single or double cords and is one of the strongest forms of hand sewing known. The method was in use in Europe as early as the 8th century, and represents the foundation upon which hand bookbinding was built and developed for a thousand years.
The number of bands, which were always double (i.e., two cords or thongs adjacent to each other and almost touching), on which 12th and 13th century books were sewn varied from two to five (in the latter case the cords being spaced so there was a greater space between the cords than between the end bands and the head and tail of the book), although examples of books sewn on as many as fourteen cords are known. The use of double cords gradually diminished, however, and by the middle of the 16th century the technique of sewing on single cords had become fairly well established, although largely for smaller books and economical bindings.
The use of flexible sewing has been dominant in fine binding until the present day; however, its use declined sharply from the end of the 18th century until the end of the 19th century, when it was revived to some extent due to the efforts of T.J. Cobden-Sanderson and Douglas Cockerell.
Flexible sewing is not suitable for books printed on very heavy paper, nor in cases where the book is made up of very thick sections. It is also unsuitable for use with coated papers. If used on small volumes, the sewing thread and, therefore, the cords, must be proportionally thinner; otherwise there will be a reduction in flexibility. (]61, 72, 236, 335, 343)