Print flashcards whose backs land on the right fronts

Because turning a sheet of paper over changes the coordinate system on the second side, and which way it changes depends on the edge it is turned about. Lay a sheet down portrait, front side up. If the printer flips it about the long edge — the tall side, like turning a page in a book — then a point on the back sits over the mirror image of the front across the vertical centre line, so the left column and the right column swap. If it flips about the short edge — top over bottom, like a notepad — the rows swap instead. Print the backs in reading order and either way the backs land on the wrong cards. This tool mirrors the backs sheet for you, and the "long edge / short edge" setting is what tells it which mirror to use. Short edge is handled by taking the long-edge sheet and turning the whole block 180°, which is the same thing mathematically and also stops the cards printing upside down — mirroring the rows alone would put the right card in the right place and print every one of them the wrong way up. Nothing in a browser can read your printer driver, which is why this is a setting with a ten-second test rather than something detected for you.

Print pages 1 and 2 of this deck, double-sided, and hold the sheet up to a light. The card number is printed on both faces for exactly this. If the number on the back sits on top of the matching number on the front, the setting is correct. If it lands on a different card, switch the setting and print those two pages again. You can usually also read it off the print dialog: the duplex option is often labelled "Flip on long edge" / "Flip on short edge", or shown as two little icons of a page turning — the book-style icon is long edge, the calendar-style icon is short edge. Long edge is the default on the large majority of printers, which is why it is the default here. Worth knowing: the setting lives in the printer driver, not in the document, so it can be different for the same printer on a different computer, and some drivers remember it per-application.

When you do not want to think about any of the above, when your printer has no duplex unit, or when you want a stiffer card without buying stiffer paper. Fold-over prints the front and the back next to each other on one side of the sheet with a dashed fold line between them. You cut the pairs out, fold each one along the line, and the card is two layers thick — which on ordinary 80 gsm office paper is the difference between a card you can shuffle and a card that flops. The cost is that you get half as many cards per sheet, and that a folded card has a visible crease down one edge. One detail worth stating because it looks like a bug: the back panel is printed the normal way up, not upside down. Folding on a vertical crease reverses that panel left-to-right, and turning the folded card over to read it reverses it back, so the two cancel out. A horizontal crease would need the back rotated 180°, which is why this tool only folds one way.

Two reasons, and both are about the printer rather than the design. Almost every inkjet and laser printer has an unprintable strip around the edge of the paper, usually three to five millimetres, where the rollers grip; artwork placed there is simply not printed, so cards laid to the paper edge would lose their outer column. And the block has to be centred on both axes for double-sided printing to work at all — the top gap and the bottom gap are equal, and the left and right gaps are equal, so flipping the paper on either edge maps the block exactly onto itself. That is why all three card sizes here produce the same 180 × 240 mm block: it is the shared geometry that makes the mirroring reliable, not a coincidence of the sizes chosen. Adding a fourth size that did not divide into that block would break alignment for that size alone, with nothing on screen to show it.

Ordinary 80 gsm office paper works for fold-over, because folding doubles it. For double-sided, the honest answer is that plain office paper is too thin: you can read the back through the front, which on a flashcard defeats the point. 160 to 200 gsm card stock is the usual choice and most home printers take it from the manual feed tray one sheet at a time — check your printer’s maximum before buying a ream, because a printer that jams on 200 gsm will do so repeatedly. Note that many duplex units refuse to take card stock automatically even when the printer can print on it fed by hand, which is another argument for fold-over if all you have is a stack of 160 gsm. Avoid glossy photo paper: it is slippery to shuffle, it shows fingerprints, and you cannot write a correction on it.

The cards touch edge to edge, so each cut line is shared between two cards and one pass of the blade trims both. Cut all the way across the sheet in one direction first, then across the strips in the other — that gives you long straight cuts against a ruler or a guillotine, rather than short ones where the blade is easiest to wander. If you are using scissors, cut just inside the printed line rather than trying to follow its centre, and accept that a millimetre of variation is normal; a guillotine cutting a stack of sheets drifts by about that much even in a print shop. The cut marks sit outside the card block, so they are trimmed away with the outer edge and never appear on a finished card. If you plan to round the corners with a punch, do it after cutting and expect to lose about three millimetres from each corner.

The type steps down automatically as the text gets longer, in fixed steps rather than by measuring the card — measuring would mean two measurements per card, four hundred of them on a full deck, and every one of them taken before the page font has finished loading and therefore wrong on the first render. So it is a rule you can read rather than a measurement you have to trust. Past the point where stepping down still helps, the text is cut off at the edge of its own card rather than spilling over its neighbour, and the form warns you which card numbers are pushing it. If you see that warning, the fix is usually to move up a card size or to shorten the back — a flashcard that needs a paragraph is a note, and it will not be read as a flashcard anyway. The "Text size" control shifts every step up or down together if you want the whole deck tighter or looser.

It is saved, and it does not leave your device. The deck stays in this browser’s local storage as you work, so closing the tab by accident does not cost you the hour you spent typing forty pairs of words. Nothing is uploaded: the whole page is static files and the sheets are laid out by your own browser, so there is no server for it to be sent to. "Start a blank deck" deletes the stored deck, and so does clearing your browser data — and because it is stored in the browser rather than in an account, it is on this device only and will not appear on your phone. One thing worth stating plainly rather than softening: this site carries Google ads, which are a third-party request the same as on any ad-supported page. They never see what you type, and they are hidden from the printed sheet.

Two hundred cards is the limit in one deck, which is thirteen A4 sheets at the smallest size and fifty at the largest. It is a limit on the page rather than on the paper: the preview lays out every sheet as a real A4 sheet, and a browser asked to hold several hundred of those at once gets noticeably slow to type into. For more than that, print the first two hundred, then use "Start a blank deck" and paste the rest — the printed cards are physical objects and do not care that they came from two sittings. If you are printing two different decks, turn on "Print the deck name on every card" before each run; a small label in the corner of every card is what stops two shuffled decks from becoming one unsorted pile.