When you pick up a booklet or a brochure, you usually don't think about how it was printed. Or at least you usually don't think about how your printer laid out the job on the press sheet to minimize waste, reduce time on press, ensure that the paper grain is in the proper direction, and facilitate post-press operations such as folding and trimming.
Think of it this way. If you are printing a brochure, the more copies of the job you can fit on a press sheet, the fewer impressions the press must make to complete the job. Since press time factors into the overall cost of a job, minimizing press time saves you money.
In terms of paper usage, the less wasted paper, the better. Not only does saving paper benefit the environment, but it also saves you money. Positioning as many copies of a job on a press sheet as possible means you are spending less money on paper that is trimmed away and discarded.
In terms of folding and binding, precise imposition can ensure that the pages of a multi-page publication, once printed flat on a press sheet, can be folded in such a way as to yield an economical press signature (i.e., the largest number of consecutive pages that can fit on a press sheet and then be folded down into one of many booklets that are stacked or nested to comprise the final publication).
The past few paragraphs touch on a distinction that I would like to more fully address. You can lay out two flat pocket-folder blanks on a press sheet of a certain size that can be printed, diecut, folded, and glued to yield two complete pocket folders. Or, if your job is a 16-page booklet, you can lay out eight pages on each side of a press sheet, print the job, and then fold and trim the page down into a 16-page booklet. If your book is 32 pages in length, you can print two signatures in the same way, nest one inside the other, and saddle stitch the booklet.
(The key here is that for a small project you can lay out multiple copies of the same job side by side on the press sheet, or for a multi-page project, you can lay out one copy of multiple pages.)
Let’s say your job contains 160 pages and is therefore perfect bound (i.e., since it's too large to be saddle stitched). In this case, you would print ten 16-page signatures (eight pages on each side of the press sheet, placed so as to fall consecutively once folded and trimmed).
Multiple press runs start to cost a lot of money. Let's assume the pages in your book are 8.5” x 11”. On a 25” x 38” press sheet, you can impose one row of four pages over another row of four pages (8.5” x 4 = 34” plus room for bleeds for the long dimension of the sheet, and 11” x 2 = 22” plus room for bleeds for the short dimension). You are pretty much using up the sheet (22” plus bleeds x 34” plus bleeds on a 25” x 38” press sheet). That's economical.
But let's say your printer has a larger press. (This 25” x 38” sheet would fit on a 40” press, but there are 50” or even 60” presses, and larger, in existence.) If you can get more pages on a sheet (let's say a 32-page or even 64-page signature) by either using a larger press sheet (on a larger press) or making the pages smaller, you can print the entire job with fewer press runs, and this will save you money.
Do the math. If your job is 160 pages in length, and your press will only yield a 16-page signature (8 pages on the front and 8 pages on the back of the press sheet), you can cut the number of press runs on a sheetfed press in half, from ten to five, if you can find a way to produce a 32-page signature (16 pages on the front and 16 pages on the back of the press sheet).
You may want to keep this in mind as you lay out a 20-page booklet. If your 8.5” x 11” job fits on your printer's 40” press, economically yielding a 16-page signature and a four-page signature, you might want to rethink your design. Otherwise, you would need to do two press runs, one for each signature, rather than just one for a 16-page signature. Granted, you can't always do this. You may have too much content. But sometimes you can adjust the layout or the type size and save a lot of money by reducing the number of press runs for a job.
Alternatively, when your printer says you can get more pages on his particular press by making each page a little smaller, take his words to heart. In some cases, all it will take is reducing the trim size by a fraction of an inch.
Here's one final reason to consider the imposition of your multi-page job. Paper folds more easily with the paper grain direction. If your pages are laid out on a press sheet (let's say a 25” high x 38” wide sheet) with the 11” dimension of a page on the short side of the press sheet (i.e., two 11” pages, one over the other), when the job is folded, the grain will run parallel to the bind edge of the booklet. That is, you will fold with the grain. (Note: This assumes you have a press sheet in which the majority of the paper fibers run parallel to the short dimension of the sheet: i.e., grain short.) Folding a signature of a booklet against the grain makes turning pages difficult, makes pages not lie flat, and causes waviness in the paper.
Paper-Folding Imposition Sample
Here's a simple way to make a miniature signature out of a sheet of paper. It will show you exactly how the printer will lay out the pages of a 16-page signature (for example). Fold the sheet in half (8.5” x 11” folded to 5.5” x 8.5”). Fold it in half again (perpendicularly) to 4.25” x 5.5”. Finally, fold it again to 4.25” x 2.75”.
You now have a little booklet with the head (top) and front (face) margins of some of the pages still attached.
Pull the pages apart slightly at the bottom, and number them consecutively. Then unfold the sheet again. You will note that the center spread (pages 8 and 9) comprises two pages side by side but that all other pages are not side by side. (Actually pages 14 and 15 are side by side as well, but they are in reverse order.)
Once the press sheet has been printed on both sides and allowed to dry, the folding and trimming equipment in the printer's post-press (or finishing) department will fold the press sheets into these little booklets and then trim off the head and face margins to free up the pages where they are still attached.
[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]