Print finishing / Folding and trimming
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.
What is physically going on
A folded sheet carries pages that are not consecutive: the outermost sheet of a booklet holds the first page and the last on one side. Working out which page goes where, and mirroring it for the back, is imposition, and it is done by the printer with tooling built for it. A designer who supplies reader spreads, pages paired left and right as they will be read, has done a different and unhelpful rearrangement: the printer now has to split them apart before imposing, which is extra work and an extra place for an error. Single pages in reading order is what imposition tooling wants, and it is also what a renderer naturally produces.
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.
- Supply single pages in reading order unless the printer asks for something else. That is the input their tooling is built for.
- Ask before assuming. Some workflows do want spreads, and a one line question at the start removes the whole risk.
- Number the pages in reading order and let imposition handle the physical order. Never renumber to match a sheet layout.
- Where a design genuinely spans two facing pages, supply it as two single pages that meet at the trim, and tell the printer it is a crossover.
- Keep the page size constant through the document. A mixed-size document complicates imposition and is a common cause of a wrong proof.
- Remember that a document rendered from HTML naturally produces single pages in order, so this is mostly about not doing extra work rather than about doing any.
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.
/* Single pages, in reading order, one size throughout. This is the
input imposition tooling expects, and it is what a renderer produces
naturally, so the work here is not doing anything extra. */
@page {
size: A4;
margin: 22mm 20mm 26mm;
}
/* A crossover: a design meeting across two facing pages. Supply it as
two single pages that meet at the trim, and tell the printer. */
.crossover-left { break-after: page; }
.crossover-right { break-before: page; }
/* Not this: pairing pages into reader spreads by using a double-width
page. The printer has to split them again before imposing.
@page { size: 594mm 420mm; } <-- two A4 pages side by side */What people do instead
Exporting reader spreads because that is how the design was reviewed. Review and supply are different artefacts: the spread view is for people, and the printer needs the pages. Sending the review artefact to the press adds a manual step at the point where mistakes are most expensive.
How to find out rather than assume
Open the supplied file and confirm each page is one page at the finished size, in reading order, with a page count that matches the extent. Then ask the printer to confirm the format is what their workflow wants before the run rather than after.
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.
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.
Blank pages in a double-sided document, and making them deliberate
The empty sides that appear in any double-sided document, which are either chosen by you or produced by accident.
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.
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.