Print finishing / Folding and trimming
Creep in a folded booklet, and the margin that shrinks in the middle
The way inner sheets of a folded booklet push outwards past the outer ones, so trimming the stack flush takes more off the inner pages than the outer.
What is physically going on
Paper has thickness. Fold several sheets inside each other and each inner sheet has to travel around the ones outside it, so its outer edge protrudes further than the sheet outside it. Trim the finished booklet flush and the innermost pages lose more from their fore-edge than the outermost do. The visible result is a margin that is even on the outside pages and progressively narrower toward the middle, and if the design ran anything close to the fore-edge, the middle pages lose it. The compensation, moving the content inwards by a progressively larger amount toward the centre, is done during imposition, not in the document, which is why the document's job is to leave enough room for it to be unnecessary.
The decisions
Reasons rather than a description of the code. Where a number depends on the stock or the press, the instruction is to get it from the printer rather than to quote one here, because spine width, creep and bulk are not constants.
- Keep a generous fore-edge margin on any document that will be folded and trimmed, so creep eats slack rather than content.
- Get the creep allowance from the printer rather than estimating. It depends on stock thickness and page count and it is theirs to compute.
- Do not try to compensate for creep in the document. It is applied at imposition and doing it twice is worse than not doing it at all.
- Avoid a page border or a frame on a document that will be saddle stitched, because uneven trim makes the frame visibly wander between the outer and inner pages.
- Keep folios well inside the fore-edge, since they are the element closest to it and the first to be clipped.
- The effect grows with extent and with bulk. A thin booklet on light stock has almost none, and a thick one on heavy stock has a lot.
What it means for the page rule
The page rule is the part of this a document author controls. The binder decides the gutter and the printer decides the creep allowance; the document's job is to leave the room those decisions need.
/* The document's job is slack, not compensation. Creep is applied at
imposition; doing it here as well applies it twice. */
@page {
size: A4;
/* A generous fore-edge, so an uneven trim takes margin rather than
content. The bound edge is handled separately for the gutter. */
margin-top: 22mm;
margin-bottom: 26mm;
}
@page :left { margin-left: 24mm; margin-right: 30mm; }
@page :right { margin-right: 24mm; margin-left: 30mm; }
/* Avoid on a saddle-stitched document: a frame makes uneven trim
visible, because the border moves relative to the paper edge between
the outer pages and the middle ones.
.frame { position: fixed; inset: 12mm; border: 1pt solid #000; } */What people do instead
Running a page border on a saddle-stitched booklet. Every other design element tolerates a wandering trim; a border is a straight line at a fixed distance from the edge, which is precisely the thing that makes the wander measurable to the eye.
How to find out rather than assume
Ask for a folded and trimmed dummy at the real extent on the real stock, then measure the fore-edge margin on the outermost and innermost pages. The difference is the creep. If content is inside that difference anywhere, the margin is too tight.
Where this API sits in it
Outside it. This API renders HTML to PDF: it produces a sequence of pages at the size and margins your CSS and the render options set, and everything on this page happens afterwards, to paper, in a bindery. There is no imposition option, no binding option and no finishing option, and there could not be, because none of those are properties of a file. What the render does decide is the two things finishing cares about most: how many pages there are, and how much room is left at the edges. Both of those are set before anything is printed, which is why a finishing constraint discovered late is expensive and one designed for is free.
Frequently asked
Can the API impose the pages into printer spreads?
No, and you should not want it to. Imposition is done by the printer with tooling built for it, against the specific press, stock and folding scheme, and the input that tooling wants is single pages in reading order, which is what a render naturally produces. Supplying anything else means somebody has to undo it first.
How do I get the page count to a multiple of four?
By deciding where the padding goes rather than letting it land at the end. Add the blanks deliberately, style them with the blank page rule so they carry no folio or running header, and mark them as intentionally blank if the document is formal. Counting the covers into the total is the step people miss.
Do any of these apply to a document that is only ever read on screen?
No. This cluster is about physical operations on paper, so if a document is genuinely never printed, none of it is your problem. It is worth checking that assumption rather than making it: delivery notes, permits, method statements and invoices all end up on paper more often than the people generating them expect.
Related finishing topics
The operations that constrain each other, then the rest of the same group.
Why a stapled booklet has to have a page count divisible by four
A booklet made by folding sheets and stapling through the spine, where every sheet carries four pages and the arithmetic is therefore not optional.
Supplying single pages, and letting the printer impose them
The rearrangement of pages into the order they have to be printed on a sheet, which is the printer's job and is made harder by supplying pages already paired.
Why the panels of a folded leaflet are not all the same width
A sheet folded into panels, where any panel that folds inside another has to be narrower than it so the fold closes.
Bleed and crop marks, as a layout problem
The block in the document that this constraint lands on.
Every finishing topic
The full list, grouped by binding, folding and trimming, and collation.
None of this is a render option, and none of it could be. What the render controls is that the page count and the margins are the ones the finishing needs.