Two Ways to Apply Creep, and Why One Eats Your Trim

Fold a stack of sheets in half and the inner sheets stick out at the fore edge. The thicker the paper and the more sheets you nest, the further they stick out. Trim the folded block square and the inner pages lose more of their outer margin than the outer pages do — on a fat booklet, enough to take a page number off.

Creep compensation moves content inward, toward the spine, in proportion to how deep a sheet sits in the nest, so that after folding and trimming every page has the same margin. The total is roughly caliper × nested sheets, and it is distributed linearly: zero on the centre spread, maximum on the outermost.

That much is standard. What is not standard, and what nobody writes down, is what actually gets moved.

An imposer has two ways to apply that shift, and they behave completely differently when the sheet is tight.

• Shift the leaf. Move each whole page — its trim box and everything in it — outward on the sheet. Simple, and correct when the sheet is bigger than the pages. It needs somewhere for the leaf to go.
• Shingle the artwork. Leave the cell where it is and slide the CONTENT inside it toward the spine. The trim boxes stay on their grid; only the ink moves.

On a generous sheet the two produce the same folded result. On a tight one they do not, and the default sheet for a booklet is as tight as a sheet gets.

Ask an imposer for a booklet and it will usually choose a sheet that is exactly two pages wide — two A5 leaves make a 296 × 210 mm sheet, and that is the efficient answer. There is no spare width at all.

Now tell it to shift each leaf outward. There is nowhere to shift to. Something has to give, and what gives is the finished size: the imposer caps the trim to whatever is left after reserving room for the shift, then clips each leaf to the smaller cell.

Measured on a 16-page A5 booklet at 2 mm of creep per sheet:

Three millimetres of finished width, gone, on a booklet that was supposed to trim to A5. The artwork starting at −1.5 mm is worse than it sounds: that strip is outside the sheet, so it is not moved, it is deleted. And because the clip happens per leaf, it happens on both edges of every page.

Four sheets at 2 mm gives 6 mm of total shift; the trim is capped at 148 − 6/2 = 145 mm and each leaf's artwork starts 6/4 = 1.5 mm off the edge. The numbers fall straight out of the model. Nothing is malfunctioning — the model is simply the wrong one for this sheet.

The symptom is easy to miss because the creep number you typed is the number you get. Check the consequences instead:

1. Measure between the crop marks on an inner sheet. They sit on the trim. If they are closer together than your finished size, the imposer has taken the creep out of your product.
2. Look at the fore edge of an inner leaf at high zoom. Under a shifting model with a tight sheet, content is clipped flat at the cell boundary — a hard vertical edge through the artwork, not a margin.
3. Set creep to zero and compare the finished size. If it changes, the creep is coming out of the trim.
4. Give the sheet room. If your imposer only shifts leaves, choose a sheet wider than two pages by at least the total creep. The shift then has somewhere to go.

Booklet shingles the artwork. The cell stays where it is, the trim stays the size the sheet implies, and the content moves inside it — which is what creep compensation is for. The finished A5 stays A5 whatever the page count.

The panel also states the figure that matters, which is not the one you typed. A per-sheet creep of 0.4 mm on a sixteen-sheet book is 3 mm of movement between the outermost spread and the innermost, and 3 mm is the number to check against a folded dummy:

SRA4 landscape · saddle stitch · 16 sheets, 32 faces · 0.4 mm creep per sheet · artwork shingled up to 3.0 mm toward the spine

There is a limit to how far artwork can be shingled before it runs past the fold, so an absurd request is reduced and the reduction is named rather than applied quietly.

One thing PDF Press does not have is a paper-stock field. Creep depends on caliper, and caliper depends on coating, bulk and finish in ways a gsm figure does not capture — so the suggestion comes from the page count, and the millimetre field is there for you to type the measured figure into. Measure the stock with a micrometer, multiply by the nested sheets, divide by sheets − 1, and type that.