Numbers on the back, and the one setting that puts them there
Page two of every PDF is the same grid with no photo on it, one number per cell. Print it double-sided and each piece carries its own position. Get the duplex setting wrong and the numbers land on the neighbouring column — which looks like a design fault and is actually a paper-handling one.
The back sheet is not a design flourish. It is the answer to the least fun moment in the hobby: forty-eight pieces face-down on a table, every one of them the same warm brown, and no idea which edge is the sky. With numbers on the reverse, a piece you cannot place by colour can be placed by arithmetic.
What the back sheet is
Same grid, same cut lines, no photo. Instead each cell holds a number, and the numbers run 1 to N left to right, top to bottom — straight through, in the order the back sheet reads when you hold it in front of you.
Here is a measured example. A 4 × 3 grid renders its back like this, and this is read out of the actual sheet geometry, not described from memory:
| Row | Column 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Top | 1 | 2 | 3 | 4 |
| Middle | 5 | 6 | 7 | 8 |
| Bottom | 9 | 10 | 11 | 12 |
Verified by extracting the text positions from the generated SVG and sorting them by x. Number 1 sits behind the top-left piece, number 12 behind the bottom-right — the same order as the front grid.
The numerals are sized to the cell, roughly a tenth of its width per digit pair, so a 31.7 mm cell gets digits you can read from across a table and a 15 mm cell still gets something legible. Each digit is drawn with a paper-coloured halo behind it, because a numeral printed straight onto card picks up the texture underneath.
Why it has to be mirrored
This is the part that makes the feature work, and the part that is easy to get wrong.
When a printer duplexes, it turns the sheet over. Print an identical image on both sides and the back comes out offset by a full page width — number 1 behind the top-right piece instead of the top-left. The back artwork therefore has to be mirrored about the grid centre before it is sent to the printer, so that the flip cancels the mirror and the numbers land where they belong.
The mirror axis is the centre of the picture block, not the centre of the paper. Those two are not the same point, because the block sits inside a 10 mm margin and above an 18 mm footer strip. Mirroring about the paper centre puts every number a few millimetres off — small enough to look like sloppy typesetting rather than a wrong axis.
The duplex setting, in one table
| Setting | Result | Notes |
|---|---|---|
| Flip on long edge | Correct | The book-style flip. This is what the back sheet is drawn for. |
| Flip on short edge | Rows reversed | The numbering runs upside down relative to the front — numbers 9–12 sit behind the top row. |
| Print sides separately, by hand | Correct, if careful | Print page 1, put the stack back in the tray, print page 2. Feed the same edge both times. |
| Single-sided only | Back on a second sheet | Keep it as a reference grid, or glue it to the back. |
“Long edge” means the long side of the sheet is the hinge, which for a portrait page is the left side. Some drivers call it “long-edge binding”, “flip on long side” or simply show a diagram.
If the numbers are offset
A little drift is normal and mostly harmless: the numeral is centred in its cell, so a 2–3 mm registration error still leaves the number behind its own piece. Judge by whether the number is nearer the right cell than the wrong one, not by whether it is perfectly centred.
| What you see | Cause | Fix |
|---|---|---|
| Offset by roughly half a page width | Wrong flip edge | Switch to flip on long edge |
| Offset grows towards one corner | Sheet scaled, not 1:1 | Check the 100 mm bar |
| Whole grid shifted up or down a few mm | Paper fed from a different edge | Feed both passes from the same edge |
| Numbers run bottom-to-top | Short-edge flip | Switch to long-edge flip |
| Numbers are mirrored shapes | Back artwork printed without the pre-mirror | Print page 2 of the generated PDF, not a copy of page 1 |
Three ways to use them
1. Sort before you start
Turn all the pieces number-up and lay them in rows in sequence. You have just reconstructed the picture without looking at it once. Flip the board and solve the puzzle you already know is correct. This is the workflow that makes a printable puzzle feel like a real one, and it is the only way a 200-piece A4 sheet stays fun.
2. Find the missing piece
When one piece is gone, the gap tells you the number. Count the row, count the column, and you know it is piece 37. Tidy up, and the piece you are looking for is the only one left on the floor with 37 on it.
3. Let a child build it alone
Number-up assembly needs no picture recognition at all — only counting. A six-year-old who cannot yet hold the whole image in their head can still finish a 48-piece puzzle unaided, and the flip at the end is the reward. This is why the back sheet is on by default.
?back=0 to the URL, and you get a front sheet only. ?num=alpha adds letters alongside the numbers — two characters per cell, which halves the number of pieces that share a label and makes sorting faster.
Print both sides, in one pass
The generator produces a two-page PDF: the front, then the numbered back. Choose flip on long edge and each number lands behind its own piece.