Print finishing / Folding and trimming
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.
What is physically going on
Divide a sheet into three equal panels and fold it into a letter fold and it will not close: the panel that tucks inside has to travel around the fold, so it needs to be shorter than the panels outside it. The same applies to every scheme where a panel nests inside another, and by more when the stock is heavy. Equal panels are the intuitive division and they are wrong for every fold except a simple half. The amount is small, a couple of millimetres, and it is enough that a leaflet with equal panels does not sit flat and its inner panel is creased at the wrong place.
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.
- Make any panel that folds inside another narrower than the ones outside it, and take the amount from the printer for the stock.
- Draw the panel widths as page geometry rather than as content margins, so the folds fall where the design says and not where the content ended.
- Design each panel as its own composition. A reader sees one panel at a time while unfolding, not the flat sheet.
- Keep content clear of the folds. A crease through text is unreadable, and a crease through an image is a visible line.
- Remember the flat sheet is not the reading order. Which panel a reader sees first depends on the fold, and it is easy to lay out a leaflet whose cover is on the wrong panel.
- Confirm the scheme with a folded dummy before the design is finished, because folding schemes are much easier to understand from paper than from a diagram.
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.
/* A letter fold on A4 landscape. The panel that tucks inside is
narrower, so the fold closes flat. Take the difference from the
printer for the stock: it grows with thickness. */
@page {
size: 297mm 210mm; /* A4 landscape, folded into three */
margin: 0;
}
.leaflet {
display: grid;
/* outer, outer, inner: the tucked panel is the narrow one */
grid-template-columns: 99mm 99mm 97mm;
height: 210mm;
}
.panel {
/* content clear of the folds on both sides */
padding: 14mm 10mm;
}
/* Each panel is its own composition: a reader sees one at a time while
unfolding, not the flat sheet. */What people do instead
Dividing the sheet into equal panels. It is the obvious division, it is what a grid gives you by default, and the folded result does not close flat: the inner panel's edge fouls the fold and the leaflet springs open.
How to find out rather than assume
Print the flat sheet at actual size, fold it by hand the way the machine will, and see whether it closes. Then unfold it slowly and check that the panel you meant as the cover is the one facing you at the start.
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.
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.
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.
Tab dividers in a printed manual, and where the tabs actually sit
Divider sheets whose tabs extend beyond the edge of the document, cycling down the page so that every tab is visible at once.
Fold and punch guides, 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.