The flat back, which was necessary for the same reason in the case of books bound at a later date in velvet, cloth, silk or canvas, necessitated some modification of the thick projecting bands of the ordinary book, and the requisite flatness was attained by using strips of vellum or tape instead of cords of hemp. Then it was found that it was not necessary to fasten the back of the cover to the back of the sections, so the “hollow back” came into existence. [Illustration: FIG. 31.--Book bound with “open” or “hollow” back, and modern headband, cut off at each end.] The majority of modern books are bound with hollow backs; it can be recognised by opening a book to the full and seeing if the back is separate, and it has one real virtue as well as several vices. If a book should happen to be printed too far back, a hollow back binding will enable it to “throw up,” and show the printing right down to the inner edge of the paper, whereas a flexible binding always tends to open less freely, especially in the middle. For books that are likely to be much used hollow backs are unadvisable as they are sooner worn out, but for small light books there is no doubt much to be said in their favour. [Illustration: FIG. 32.--Back of book prepared with cut trenches to hold the bands.] Among English binders of note I think Charles Lewis was the first to use hollow backs extensively. But for fine books there is no doubt that the old-fashioned flexible sewing on raised bands is the best in the long run. Books bound with hollow backs often have the bands “sawn in,” that is to say, a trench is cut for each band across the back of the sections. In these trenches the bands are laid, and the sewing is of a simpler and quicker sort than it is when the thread is brought round each band as it is in the flexible style. Moreover, there is a weak point where the thread touches the edge of the saw cut, and at this place the paper is always apt to give. It is obvious that to cut away paper from the back of a section must always be not only a barbarism but also structurally wrong. Such a method of sewing a book can only be excused on the score of cheapness, and it may be urged that in this case it does not matter.