Anti-Knight Sudoku

Anti-Knight Sudoku borrows a rule from chess. On top of the normal Sudoku rules it adds one more: no two cells a knight's move apart may hold the same digit. A knight leaps in an L-shape — two cells one way and one cell across — so every cell now "attacks" up to eight others scattered around it, and none of them may copy its value.

The rule: a knight's move can't repeat

Place a digit and it locks that digit out of eight cells: the eight knight's-move landing squares around it. In the diagram, the amber 5 in the centre forbids a 5 in every red cell — the classic knight pattern. All the usual rules still apply, so the 5 also rules out its row, column and box as normal; the knight constraint is stacked on top.

What the extra reach buys you

Those eight forbidden cells sit outside the digit's own row, column and box, which is exactly why the constraint is so useful: it forbids repeats where ordinary Sudoku is silent. A 5 in one box now reaches into neighbouring boxes and knocks the 5 out of specific cells there — often enough to turn a stuck box into a hidden single. Near the edges and corners a cell has fewer than eight knight targets (a corner cell attacks just two), so the effect is strongest in the middle of the grid.

Combined with the ordinary rules

Think of each placed digit as casting two shadows: the familiar row/column/box shadow, and a scattered knight's shadow eight cells wide. When you are hunting for where a digit can go in a box, remember to knock out not only the cells its box-mates see, but also every cell a knight's move from an existing copy of that digit anywhere nearby. That second sweep is what makes Anti-Knight grids solvable with far fewer givens than Classic.

Solving strategy

• Add a knight sweep to every placement. After writing a digit, immediately cross it out of the eight knight cells around it.
• Work the centre first. Central digits attack a full eight cells, so they resolve the most.
• Use it for hidden singles. The extra reach frequently leaves a digit only one home in a nearby box.
• Remember it is a repeat ban, not a placement. A knight's move only forbids the same digit, never forces one.

Common mistakes

• Using a king's move instead. The knight leaps in an L (2+1), not to an adjacent cell — mixing it up with the anti-king rule forbids the wrong cells.
• Forgetting the edges. Cells near a border attack fewer than eight cells; don't invent knight targets that fall off the grid.
• Dropping the normal rules. Rows, columns and boxes still apply; the knight constraint is an addition, not a replacement.
• Treating it as a forcing rule. It only says two cells can't share a digit — it never places one on its own.

Can I play it here?

Not yet — Anti-Knight Sudoku isn't one of the puzzle types SudokuStreak generates for now. But you can learn its rules above, and there's plenty here to sink your teeth into meanwhile:

• Try the closely related X Sudoku, which you can play right now.
• Browse all the Sudoku variants we do offer.
• Or print free puzzles from the printable PDF page.