Overlap is what stops a white line showing along your seams when a cut is not perfect. It does that by printing a strip of the neighbouring tile's content into the margin band of each sheet, so the same image lands on both sides of every seam and a short cut has something to land on. It is set in the export preview, alongside the margin, the cut lines and the page numbers.
Use 50% unless you have a reason not to. The rest of this page is that reason.
What the strip is for
Without overlap, every seam depends on cutting exactly on the line and placing the sheet exactly against its neighbour. A cut that lands a millimetre inside the line leaves a sliver of bare paper, and against a dark or photographic poster that sliver reads as a white line.
With overlap, the sheet to the right carries a copy of what was on the sheet to its left. Cut short and the gap is filled by that copy, because the same image is printed on both sides of the seam.
The amount is a fraction of the margin
This is the part that catches people out, and it is worth being precise about. Overlap is a percentage of your margin, not an absolute distance.
At a 0.50 in margin, 50% overlap gives a 0.25 in strip on the left and top edge of every interior tile. Change the margin and every amount changes with it.
If your margin is 0, your overlap is 0 — whichever percentage you pick. There is no band for the strip to print into. Set a margin first; 0.50 in is the recommended value and it also prevents cropped pages.
Measuring it this way is deliberate. A strip stated as a length would have to be re-checked every time you changed paper size or margin, and could be set wider than the band it prints into. A fraction is always valid.
Why not 100%
100% runs the strip all the way to the physical paper edge, which most consumer printers clip — the same unprintable border the cropped pages article describes. It is deliberately not capped, because borderless printers exist and the choice is yours. It is not the value to reach for first.
Which edges get a strip
Strips are printed on the left and top edges of each tile, the side opposite the scissor marks.
The outer perimeter gets none: the first column has no left neighbour and the first row has nothing above it. That is expected, and those edges are trimmed to the poster boundary anyway.
That one-sided arrangement is what makes assembly work. Cut each sheet on its right-hand line and lay it over the sheet to its right, so the upper sheet's cut edge hides the lower sheet's strip and any gap under it. Work left to right along a row, then top to bottom down the rows. There is more on this in assembling your poster.
It is a paid feature
Overlap requires a Pro or Pro+ plan. Setting it on a free account reverts it to Off and opens the upgrade prompt, so it is worth knowing before you plan around it — see pricing.
Margins, cut lines and page numbers are available on every plan; overlap is the one control in that group that is not.