Modern books are fastened together by sewing; as a rule thread is used, and always in sewing good books. Thread sewing is very much better than wire stitching. In the first place, thread has not the disadvantage of rusting, to which wire is always subject; secondly, thread does not break the paper in the back, as so frequently happens when cheap paper containing much wood fiber is wire stitched. The greatest advantage of thread sewing over wire lies in the flexibility of the spun thread; with sharp pressure it lies flat in the sheet, whilst wire does not give at all. Besides, it is flexible the whole length, and this adds greatly to the life of the book.

All thread sewing is now done by machinery and is really satisfactory. Only the very smallest job shops sew their books by hand.

The principle of sewing is to fasten each sheet to several cords or bands by means of a long thread running right along the inside of the sheet. These cords give the book its hold to the cover; therefore the more cords used, the more firmly is the book secured to its cover. There are now two methods of fastening the sheets on the cords; first, there is the older method of passing the thread along and out of the sheet, around the cord, and again into the sheet, and repeating the same movement at the next band.

Nowadays, this method is almost entirely discarded. When for special reasons, or on account of its greater strength, it is desired to imitate the old method, the sewing is done on double cords; that is to say, for every cord two cords are stretched alongside each other and regarded as one. This sewing is more tedious, as the thread must take up each cord as shown in Fig. 22.

These somewhat elaborate modes of sewing have been simplified in recent years by making saw cuts in the back, in which the cords are laid. By this means the sewing thread never actually passes out of the sheet, but is drawn behind the cord lying in the saw cut and thus holds it.

That is why we "saw in" our books. The sawing-in is done with a broad saw; the so called "tenon saw" being the one most generally used. The saw cut must correspond exactly to the thickness of the cord to be used, should be less deep than wide, and should not take up too much glue when gluing up, as this might easily turn brittle. By inclining the saw to right and left alternately during sawing, the resulting cut will be something like this; this is the best and most usual form.

The common practice of widening the cut by means of a coarse file gives a triangular cut. Like this which is objectionable, as the groove gets filled with glue, thus rendering the thread liable to break. As many cuts must be made in the book as there are cords to be used besides the so called kettle stitch at head and tail by which the thread is passed from one sheet to the other.

Dividing the back for sawing in is done by marking off 1 cm from the head and twice as much from the tail and dividing the rest into equal portions with the dividers. An octavo should never be sewn on less than four cords and a folio on six. If obliged to use less through low prices, then three must be taken as the minimum number and that only in exceptional cases. If the books are very small as for instance, prayer books and hymn books less than 7 cm in height it may then be permissible to sew on two cords. The division for the different sizes for sawing in is made as shown in Fig. 23.

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