A4 is 210 × 297 mm, which makes it easy to reason about in multiples. Two sheets across in portrait gives 420 mm of width before trimming, roughly A2. Three gives 630 mm. Four gives 840 mm, which is about A0 across. Because A sizes halve neatly, an A4 tiling lands close to a standard poster size without much arithmetic.
This preset works from sheets across rather than from a target width, which is the easier way round when the paper is what you have and the finished size is negotiable. Say two sheets across and the number of sheets down follows from the shape of your image. A 4000 × 3000 photo across two A4 portrait sheets comes to 6 sheets (3 across × 2 down). Turn the paper landscape and the same picture comes to 9 sheets (3 across × 3 down), which is often fewer sheets for a wide image.
Overlap is the part that makes it work. Trimming is on the printed cut lines, one edge per sheet, and the repeated strip of image gives you room to slide the sheets until they match before taping.
One setting outside this tool decides whether any of it lines up: the print dialog. Fit to page shrinks every sheet slightly and the joins drift by a millimetre each time. Set scale to 100 percent or actual size.
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