Although the principle of backing was known at the early time mentioned, it was not universally understood and practised until quite recent times. Now, however, it is fully recognised as one of the most important processes in the binding of books, especially large ones. We have seen that the book sewn in leather bands has the ends of these bands left loose, projecting in the case of a large book some two or three inches. Holes were now carefully cut along the back edge of the board to fit the ends of the bands, sloping upwards, and some little way in other slots were cut from the upper surface of the board to meet them. Into these holes the ends of the bands were drawn, and when in proper position they were pegged down with one or more small wooden pegs. Sometimes the bands were drawn right through the boards and fastened inside. Numbers and numbers of instances of this work exist and are quite sound to-day. But such books do not open satisfactorily, as there is a disagreeable pull upon the outer sections when the book is opened. In fact, the junction between the bands and the boards is not scientifically correct, because the backs are not rounded. [Illustration: FIG. 45.--Half-bound book.] The vulnerable part of the binding will now be seen to be the soft threads which cover the bands where they adjoin the back of the sections of the book, and to protect these delicate threads a strip of leather was cut, damped and pressed over the bands so as to fit quite closely, and fastened on with glue, projecting a short way over on to the board itself so as to cover up the holes used for the bands. This is called a “half-binding.” On the leather of such half-bindings there is usually some blind tooling, lines or rolls or even small cameo stamps. Mediæval bindings are commonly provided with clasps. The original reason of this was to help to keep the vellum leaves flat, but of course artistic binders saw that clasps might be made very ornamental, and so many of them are. They have survived as ornamental adjuncts to a binding until the present day, although there is no necessity for them. [Illustration: FIG. 46.--Mediæval book with bosses, corner pieces and clasps.] Bosses in the centre and at the corners of mediæval bindings were of structural use, as they protected the actual boards of the book from wear. In early days books were kept upon their sides and probably had flat boards between each volume. So the bosses took the wear and preserved the books. Many of the mediæval bosses and corners have ornamental settings; they are generally of brass, but sometimes of silver or wood. The titles of such books were sometimes written on the front edges and sometimes on a slip of paper fastened under horn on the upper board, and in the late fifteenth century they were sometimes lettered in gold or blind, also on the boards.