All Sudoku Variants
9×9 · Constraint

Anti-King Sudoku

Also known as King's Move Sudoku

Classic rules plus a chess rule: no two cells a king's move apart — including diagonally — may hold the same digit.

5 1 9

An example Anti-King puzzle: no cell a king's move from the 5 (the shaded ring) may also be a 5.

Anti-King Sudoku adds a chess rule to the ordinary grid: no two cells a king's move apart may hold the same digit. A king steps one square in any direction — including diagonally — so every cell now "attacks" the eight cells immediately around it, and none of them may repeat its value. It is a close relative of Anti-Knight Sudoku and a building block of the famous Miracle Sudoku.

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

Place a digit and it is locked out of all eight surrounding cells. In the diagram, the amber X forbids an X in every red cell — its full ring of neighbours. The ordinary rules still apply too, so the digit also clears its row, column and box; the king constraint stacks on top.

X ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗

The centre X forbids that digit in all eight surrounding cells — the king's move.

The diagonals are what's new

Four of a king's eight neighbours — up, down, left and right — are already partly constrained by the ordinary rules, but the four diagonal neighbours are not: two diagonally adjacent cells often sit in different rows, columns and boxes, so plain Sudoku lets them repeat. The anti-king rule closes that gap. Its most quoted consequence is that no 2×2 block anywhere on the grid can contain the same digit twice — a handy check while solving.

Combined with the ordinary rules

Think of every placed digit as clearing not just its line and box, but a tidy 3×3 halo around itself. When you are working out where a digit can go in a box, remember to knock it out of the eight cells touching any existing copy nearby. That extra diagonal reach is what lets Anti-King grids be solved from fewer givens than Classic, and it combines with the anti-knight and non-consecutive rules to power the two-clue Miracle Sudoku.

Solving strategy

  • Sweep the halo on every placement. After writing a digit, cross it out of all eight surrounding cells.
  • Watch the diagonals. The diagonal bans are the ones ordinary Sudoku misses — they do the real work.
  • Use the 2×2 rule. No 2×2 block may repeat a digit; scan for near-repeats as a quick check.
  • Feed the grid. Each forbidden cell tightens a box, often leaving a digit a single home.

Common mistakes

  • Confusing it with the knight. The king reaches the eight adjacent cells; the knight leaps in an L to farther cells. Anti-King uses the adjacent eight.
  • Only checking orthogonally. The diagonal neighbours are the whole point — do not skip them.
  • Treating it as a placement rule. It only forbids a repeat; it never forces a digit on its own.
  • Dropping the ordinary rules. Rows, columns and boxes still need 1–9; the king constraint is an addition.

Can I play it here?

Not yet — Anti-King 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: