After the edges have been treated by whatever process, there follows what is termed the "backing" of the book. The volume is pressed between iron clamps, and the back is hammered or rolled where it joins the sides, so as to form a groove to hold the boards forming the solid portion of the cover of every book. A backing-machine is sometimes used for this process, making by pressure the joint or groove for the boards. Then the "head-band" is glued on, being a silk braid or colored muslin, fastened around a cord, which projects a little above the head and the tail, at the back of the book, giving it a more finished appearance. At the same time, a book-mark for keeping the place is sometimes inserted and fastened like the head-band. This is often a narrow ribbon of colored silk, or satin, and helps to give a finish to the book, as well as to furnish the reader a trustworthy guide to keep a place--as it will not fall out like bits of paper inserted for that purpose. Next, the mill-boards are applied, cut so as to project about an eighth to a quarter of an inch from the edges of the book on three sides. The book is held to the boards by the ends of its cords being interlaced, _i. e._: passed twice through holes pierced in the boards, the loose ends of the cords being then wet with paste and hammered down flat to the surface of the boards. The best tar-boards should be used, which are made of old rope; no board made of straw is fit to be used on any book. Straw boards are an abomination--a cheap expedient which costs dearly in the end. The binder should use heavy boards on the larger and thicker volumes, but thin ones on all duodecimos and smaller sizes. Next, the books are subjected to a second pressing, after which the lining of the back is in order. Good thick brown paper is generally used for this, cut to the length of the book, and is firmly glued to the back, and rubbed down closely with a bone folder. A cloth "joint," or piece of linen (termed "muslin super,") is often glued to the back, with two narrow flaps to be pasted to the boards, on each side, thus giving greater tenacity to the covering. If the book is to be backed so as to open freely, that is, to have a spring back or elastic back, two thicknesses of a firm, strong paper, or thin card-board are used, one thickness of the paper being glued to the back of the book, while the other--open in the middle, but fastened at the edges, is to be glued to the leather of which the back is to be made.