Fig. 25.
When so divided, each pair may be imposed in exactly the same order as in the four page form. In this case the paper is cut to exact size and pages one and four are printed on one side. When the ink dries, it is run through the press again and pages two and three are printed on the other side, completing the work.
An eight page form, or a sheet quarto, for a sufficiently large press, may be imposed after the work-and-turn method as in the case of the four page form. The numbered rectangles in Fig. 25 show the arrangement of pages in the work-and-turn method of imposing an eight page form. It will be observed that pages in the outside section of an eight page form are in the same order as those of a four page form. For a press that can accommodate only four pages, the inside and outside sections are imposed exactly in the same order as in Fig. 25, but are used as separate forms.
When we undertake the imposition of an eight page job, we meet the problem of margin for trim, which has not been encountered in the previous forms. In an eight page piece of work, there must be two folds and since the sheets are folded after they are printed and before they are cut, it is evident that some allowance must be made for trimming the fold at the top of the pages. Since in folding, the edges are probably not even, it is necessary to trim them also. Thus it is clear that allowance must be made for trim around each pair of leaves. Fig. 25 represents a half sheet of standard 25 × 38 paper laid on the eight page form imposed by the work-and-turn method. The line XX indicates where the printed half sheet is cut before folding. The dotted lines indicate the margins allowed for trim after folding. The method of measuring for the imposition is as follows: Suppose the type pages to be four by seven inches and centered in the pages, and the margin, after trim, to be one inch all around each page.
It is apparent then that the backs of type pages 8 and 5 are two inches from the backs of pages 1 and 4. The location of pages 7 and 6 is determined by measuring half the length of the half sheet, or 12½ inches in this case, from the front edges of type pages 1 and 4. The distance then from 8 to 7 and from 5 to 6 is 2½ inches or 15 ems pica, making ½ inch for trim. One-half of 19 inches, or the other dimension of the half sheet, measured from the foot of type page 1 to the head of 4, places the distance of 2½ inches between 1 and 4. This allows ½ inch here also for trim.
In this work, some prefer simply to fold the paper and use it for measurement, but in any case when the margins are determined, they are built in with appropriate furniture and the form is locked up. It is well to remember that, viewed from the foot of the pages in a form, odd numbered pages are always at the left of even numbered pages.
In most of the work of this character, the pages are so large that only two may be accommodated by a small press. In this case, the eight pages are imposed in pairs as follows: One and eight, two and seven, three and six, and four and five. It will be observed that the sum of the page numbers of any pair is always one greater than the whole number of pages.
PRESSWORK
