Most broken Hitori grids come from one of six mistakes, and the biggest is shading a square before it’s certain. The others are slips: forgetting the joining-up rule, shading squares side by side, thinking corners can’t touch, not circling, and shading a number that doesn’t repeat. Each one is below, with how to spot it.
If you’re new, the rules and how to solve pages explain the ideas these mistakes go wrong on.
1. Shading a square before it’s certain
When a number repeats and you’re not sure which copy goes, it’s tempting to shade one and see. We took every one of the game’s levels, shaded one square that should have stayed unshaded (always a number that repeats in its line, so it looks like a fair guess), and kept making honest moves until the game’s red mark appeared:
| Track | Moves before the red mark (median) | Still unnoticed after 10 moves | Longest |
|---|---|---|---|
| Easy 5×5 | 5 | 13 of 50 | 19 |
| Medium 7×7 | 5 | 16 of 50 | 33 |
| Hard 9×9 | 45 | 27 of 50 | 59 |
On Hard, the median is 45 moves: most of the grid. Here’s one on Medium level 3, with a wrong shade in the top-left corner:
Fifteen honest moves later, two 3s in column 5 are both left unshaded, in rows 5 and 7. Neither is anywhere near the wrong square.
How to avoid it: only shade a square when a technique makes it certain. If two copies of a number could each be the one to go, leave them and work elsewhere.
2. Forgetting the joining-up rule
The repeats are easy to keep in mind; the rule that all the unshaded squares must join up is easy to forget. But it’s what finishes most grids: cut-off (a square whose shading would cut the grid stays unshaded) is needed on 48 of the 50 Easy levels and on every Medium and Hard one.
How to avoid it: whenever the chain of moves stops, look for squares whose shading would wall something in, especially near the edges and corners.
3. Shading squares side by side
Two numbers in a line repeat, so you shade one; then a twin of its neighbour turns up, and you shade that too, right beside it. Shaded squares can’t touch side by side.
How to avoid it: as soon as you shade a square, circle the squares beside it. Then they can’t be shaded by mistake.
4. Thinking shaded squares can’t touch at the corners
They can. Only side by side is against the rules. Avoiding corner-to-corner shading by mistake makes some puzzles look impossible.
How to avoid it: remember the rule is about sharing an edge, not a corner.
5. Not circling
Shading alone leaves you holding a lot in your head: which numbers are safe, and whose twins must go. A circled number does that work for you: its twins in its row and column must be shaded.
How to avoid it: circle every square you know stays unshaded, as soon as you know it.
6. Shading a number that doesn’t repeat
The rules allow it, but it’s never the answer here: in every one of the game’s 150 levels, each shaded square has the same number left unshaded in its row or column. A number that’s alone in its row and column only needs shading if something else forces it, and in the game’s puzzles nothing does.
How to avoid it: look for repeats first. A number with no twin in its row or column can usually be left alone.
How to recover from a mistake
- A red mark means a rule is broken, but the mistake may be elsewhere, sometimes a long way back. Undo to the last move you were sure of.
- Press Stuck? It rings a wrong square before anything else, so you know exactly which one.
- Stuck with no red mark? Look for sandwiches you’ve missed, then for cut-offs near the edges. See when you get stuck.