Diagonal contact between black (shaded) cells is allowed in Hitori, and only cells that share an edge — up, down, left, or right — are forbidden from both being black. The rule comes straight from Nikoli's official Hitori description and from Simon Tatham's Singles documentation, both of which specify that black squares may meet at corners but cannot touch horizontally or vertically. This diagonal freedom matters because it changes how compactly you can pack shaded cells into the 5×5 grid: two black squares can sit on opposite corners of a 2×2 block, and that configuration is legal under the diagonal-allowed rule even though it would feel cramped if you mistakenly treated diagonals as forbidden. Knowing the rule early prevents you from leaving safe shades unshaded and helps you reason in terms of white-region connectivity instead of obsessively checking corners. If you want to confirm the rule with a working keyboard-controlled puzzle, the Hitori Puzzle tool puts a 5×5 grid under your fingers with arrow-key navigation, Space to shade, Enter for a white reasoning note, and immediate rule feedback after every move so you can verify diagonal legality in real time.
The diagonal rule in plain English
The diagonal rule in Hitori has a single, concrete meaning: two shaded cells may share a corner without penalty, but they may not share a side. If you imagine a 2×2 block of four cells, the two cells at opposite corners can both be black — they touch diagonally only and the rule is satisfied, while any pair that shares an edge within that block cannot both be black. Remove one of those cells from the black set and the remaining three are still legal, because the only thing the rule restricts is edge-sharing. The same wording is used across the official sources. Nikoli's Hitori page describes black cells as unable to touch each other orthogonally, and Simon Tatham's Singles documentation uses the same edge-only language. Both sources are explicit that diagonal contact is unrestricted.
This matters because Hitori's three constraints — uniqueness in each row and column, no edge-sharing blacks, and one connected white region — interact. The diagonal rule loosens the second constraint and lets you shade more aggressively than a first-time player might expect. In the Hitori Puzzle tool, the moment you shade a cell the validator checks the right and lower neighbors for forbidden edge adjacency and silently allows any diagonal neighbor to remain shaded, which matches the official Nikoli-derived rule set exactly.
What black cells actually cannot do
The forbidden patterns are exactly two: two black cells in the same row that are adjacent with no white cell between them, and two black cells in the same column that are stacked directly on top of each other. Anything outside those two patterns is legal, including black cells that share a corner, black cells that are one cell apart with a white cell between them, and black cells forming an L shape touching at a single corner.
In the Hitori Puzzle, validation runs after every shade. If you accidentally place two black cells side by side, the contradiction message surfaces the violation immediately. That message is a warning rather than a hard block, because pressing Space on the offending cell unshades it and the puzzle continues from the corrected state.
How to play Hitori on the 5×5 grid
- Use the arrow keys to move the selection highlight across the grid. The current cell shows a visible outline tied to the site theme.
- When the highlight sits on a number you intend to shade, press Space. The cell turns black and the validator re-runs the duplicate, adjacency, and connectivity checks immediately.
- If you change your mind, press Space again on the same cell to unshade it. The move still counts against your score, since the score formula uses the total number of shades and notes you have placed.
- To mark a cell as definitely white for your own reasoning, press Enter. A white circle appears; pressing Enter again removes the note. Pressing Space on a noted cell removes the note and replaces it with shading.
- Mouse and touch users can click to shade, while the right-click context menu toggles the white note for the same reasoning role.
- Watch for the contradiction message. It appears whenever two black cells share an edge or whenever your white region has split into disconnected islands, and it points to a deadlock in the current arrangement rather than a permanent loss.
- When every row and column shows unique numbers, no two black cells share an edge, and every white cell is reachable from any other white cell by moving orthogonally, the puzzle freezes in its solved state and your score locks in.
- To begin again, select Restart. Shades, notes, moves, score, contradictions, and completion all reset together in a single action.
Working the fixed 5×5 board to a clean 5-shade solution
The fixed puzzle in the Hitori Puzzle tool begins with deliberate duplicate pairs in every row and column and has a tested 5-shade solution: shade the first cell, the center-right cell in row two, the final cell in row three, the second cell in row four, and the fourth cell in row five. Each shade removes a duplicate from a row or column without creating edge contact, and the remaining white cells stay connected as a single orthogonal region.
You do not need to know that exact sequence to start solving. A practical approach is to scan each row for duplicates and shade the cells that are least likely to violate the no-edge rule. After every shade, look at the validation feedback, because duplicates remaining, edge pairs, and white connectivity all update together. If a shade causes a contradiction, press Space to unshade it and try the other duplicate instead. You can also use Enter to mark cells you are sure must stay white. Each note counts as a move, so use notes sparingly if you are aiming for the high-score tier.
The clean 5-shade solution finishes in 5 moves for 950 points out of a 1,000 starting score, since the score formula is max(0, 1000 − 10 × moves). Adding even a single note drops your final score by 10, because notes count toward the move total.
Score, notes, and the no-edge rule at a glance
| Action | Effect on score | Effect on validation |
|---|---|---|
| Shade a cell (Space) | −10 per move | Re-runs duplicate, adjacency, and connectivity checks |
| Unshade a cell (Space on shaded) | No further change (move already counted) | Re-runs the same three checks |
| Add a white note (Enter) | −10 per move | Note is ignored by validation; shading a noted cell removes the note |
| Restart | Resets score to 1,000 | Clears all shades, notes, moves, contradictions, and completion |
The diagonal rule sits inside this table as one of the silent passes. The adjacency check looks only at the right and lower neighbors of each shaded cell, so any diagonal neighbor of a shaded cell is automatically allowed to remain shaded. You can therefore ignore corners entirely when reasoning about which shade is safe.
When the game reports a contradiction
A contradiction message in the Hitori Puzzle tool is a deadlock warning for the current arrangement, not a permanent loss. It means one of two things is true at that instant: two black cells share a horizontal or vertical edge, or the remaining white cells have split into more than one connected region.
When the message appears, the fix is to identify the offending cell — usually the most recently shaded one — and press Space to unshade it. You can also use Enter to record a cell you have proven must stay white, then back out your last shade and try a different duplicate. Restart is available if you want a clean slate, and it clears everything at once. The validation cycle runs after every shade, which is why early contradictions are cheap to fix and why the tool reports the issue immediately rather than waiting for you to finish the puzzle.
For other deterministic logic boards with similar edge-or-diagonal rule considerations, you can try Hashi Bridges for island connectivity, Minesweeper for adjacent-mine counting, or the 15 Puzzle for a pure keyboard sliding puzzle. Each shares the same local, keyboard-first, no-account style as the Hitori solver and treats every move as auditable rather than hidden.