Anti-Knight Sudoku: rules, tips and free puzzles

Anti-knight sudoku adds a rule from chess: two cells a knight’s move apart can’t contain the same digit. There are no extra marks on the grid, so the rule is invisible until you go looking for it.

Each placed digit now rules out up to eight extra cells, and the effect ripples across box borders in ways classic solvers aren’t used to.

The rules

• Normal sudoku rules apply: 1 to 9 once in every row, column and 3×3 box.
• Two cells a chess knight’s move apart (two steps in one direction and one step at right angles) can’t contain the same digit.

How to start

Add the knight to your scan

When you look for a digit in a box, rule out cells that see the digit by row, column, box and knight’s move. The extra eliminations turn many dead ends into hidden singles.

Remember the knight crosses boxes

Most knight eliminations land in a neighbouring box, which is exactly why they are easy to miss. Make a habit of checking the boxes either side of any digit you place.

Edges and corners see less

A centre cell can reach eight knight cells; a corner cell reaches only two. Digits near the middle of the grid do the most work, so place those first when you have a choice.

Common mistakes

• Confusing anti-knight with anti-king. Diagonal neighbours are allowed to match here, unless they share a box.
• Only checking the knight moves inside the same box. Most of them land in other boxes.
• Forgetting a few of the eight directions.

How the ranks scale difficulty

Every puzzle type on Hoot’s Puzzles comes in four ranks, named after the stages of a mathematical idea. Lemma is the warm-up: about a dozen more digits are given than logic strictly needs, and singles plus the variant’s own rule will carry you through. Proposition needs a real idea or two, typically pointing and claiming. Theorem is near-minimal, so almost every clue is load-bearing and you may need pairs. Conjecture gives the fewest digits and sparser rule clues: expect to think hard.

Anti-knight puzzles have no extra clue marks, so rank is all about how many digits you start with and how deep the logic goes.

There are 99,999 puzzles per rank, each with its own number, so you can replay one or share it. Every type also has a 30-puzzle collection that climbs from Lemma to Conjecture. Every puzzle is checked to have exactly one solution that you can reach by logic alone, with no guessing.

Keep exploring

• Non-Consecutive SudokuSide-by-side cells never hold consecutive digits.
• Sudoku XBoth main diagonals are extra houses. Also called diagonal sudoku.
• WindokuFour shaded windows are extra 3×3 houses. Also known as hyper sudoku.
• Solving techniquesSingles, pairs, cage arithmetic, line logic and Latin squares, all in one guide.
• All puzzle guidesEvery puzzle type on Hoot’s Puzzles, with rules and tips.