Double-sided cards that actually line up
The usual failure in a double-sided card is the back landing on the wrong card. Mirroring the back sheet across the binding edge removes the problem entirely.
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Why the back has to be mirrored
Flip a ten-up sheet left to right and the left column becomes the right column. Print the back with the same ordering as the front and card one gets card two is reverse. Mirroring the back sheet puts every card is reverse behind the right card once the paper is turned. This tool applies that mirror for you.
Long edge or short edge
A duplex unit defaults to long-edge binding. On portrait A4 the long edges are the left and right sides, so the sheet flips left to right. When feeding by hand, match whichever way you physically turn the stack. If you are unsure, print one sheet with a big FRONT on side one and BACK on side two and watch which way it comes out.
How to proof it
Always run one duplex test on plain paper before touching good stock. Hold it up to a window and check that the front and back card outlines overlap. A drift of a couple of millimetres is workable — widen the margin to the eight-up layout, or always cut to the front side. A drift of ten millimetres or more means the binding direction is set the wrong way round.
Frequently asked questions
How many pages do I get?
One front page plus one back page per sheet. Ten cards is two pages; twenty-five cards is three sheets, so six pages. Send it to the printer with duplex enabled and the pairing works out.
Can the back use a different card size?
No — both sides use the same imposition. A back image with a different aspect ratio is scaled to the card and cropped; switch the fill mode to fit-the-whole-image if you would rather have margins than a crop.
What if I only load a front design?
The same artwork is used on both sides. That is occasionally what you want — printing both sides of a lightweight stock makes the finished card look and feel denser.