Many books are issued with plates larger than the _format_ of the book itself; they must be brought to the right size by folding, but in such a way that the folded plate is secure from injury during any subsequent trimming.
Before making a single fold in the plate, the worker should clearly see his way through his scheme for folding, so as to bring the plate to the size required with the smallest possible number of creases. Plates which are slightly wider and longer than the size of the book are easily made to fit if they are folded once or, if necessary, twice across the middle, pasting the edge of one fold on a guard and then fixing in its place in the book.
Figs. 9-13--Suggestions for folding plates and maps.
If this will not do, the plate must first be folded up from the bottom edge far enough to escape damage in trimming, and then the long side must be similarly folded. Larger plates must take more folds, always working on the principle that the length of the book should first be obtained in the best possible way, and afterwards the width is taken as the guide in making the folds.
In doing this the plate must be folded now to the front, then to the back, so that on drawing it out it opens in a zigzag fashion. For the sake of clearness we give illustrations showing the most general methods of folding. The part marked A is secured in its place in the book by mounting on a paper guard; but one may, by cutting out the map properly, leave a small margin which will serve as a guard as shown in Figs. 10, 12, and 13.
In all cases, however, it is essential that the thickness of the folded plates should be equalised by inserting guards in the back of the book.
Formerly, when several plates were inserted one after the other, it was customary to place them in such a way that they were trimmed at top and bottom alternately; now they are placed so that they are all trimmed at the top edge: this is much better, because it keeps the top edge smooth and close, thereby keeping out dust and insects. The accompanying sketches are based upon this principle.
The so-called two-page illustrations in periodicals must be treated in the same way. These are only possible in the middle of a section, where they would be caught into the back and injured if the following precaution were not observed. Such illustrations are taken out, the back edge pasted, and then placed in the back so as to adhere to the following sheets, projecting about 1/2 cm.