Anti-Knight Sudoku is a 9x9 Sudoku variant with exactly one extra rule: no two cells a chess knight's move apart may contain the same digit. A knight moves two cells in one direction and one cell at a right angle, so every cell in the middle of the grid gains eight new peers — scattered diagonally outward, mostly into neighbouring boxes. The standard row, column, and 3x3 box rules still apply in full.
What is Anti-Knight Sudoku?
Anti-Knight Sudoku borrows a single idea from chess and lays it over an ordinary Sudoku grid. In chess, a knight jumps in an L: two squares along one axis, then one square across. Anti-Knight Sudoku declares every one of those jumps to be a conflict. If a cell holds a 7, then no cell a knight's jump away from it may hold a 7 either.
What makes the variant feel different from Diagonal or Windoku is that the constraint has no shape. There is no extra region drawn on the board, no shaded window, no line to follow. The rule is a relationship between individual cells, and it points outward from wherever you happen to be looking. A digit you place in the top-left box can rule out a candidate three boxes away, and nothing on the board draws your attention to it. Learning Anti-Knight is largely learning to see that invisible reach.
Anti-Knight Sudoku Rules
- Standard Sudoku rules: Each row, each column, and each 3x3 box must contain the digits 1 through 9 exactly once.
- The knight constraint: No two cells a chess knight's move apart may contain the same digit.
- A knight's move: Two cells in one direction plus one cell at a right angle — an L shape. Up to eight such cells exist for any starting cell.
- The rule is symmetric: If A is a knight's move from B, then B is a knight's move from A. There is no direction to keep track of.
How the Knight Constraint Works
Put a marker on the cell at row 5, column 5 — the exact centre of the grid. Its eight knight peers sit at rows and columns (3,4), (3,6), (4,3), (4,7), (6,3), (6,7), (7,4) and (7,6). They form a ring around the cell with a one-cell gap between it and every one of them. Whatever digit goes in the centre is banned from all eight.
Two properties of that ring are worth internalising, because they are what the strategy below is built on.
Knight peers are never in the same row or column
Every knight's move changes both the row and the column, so a knight peer never shares a line with its origin. That means every single knight elimination is new information as far as the row and column rules are concerned — nothing is duplicated.
Some knight peers share a box — and those are the boring ones
A two-and-one offset still fits inside a three-by-three area, so a handful of knight peers land in the same box as their origin. The cell at row 1, column 1 and the cell at row 3, column 2 are a knight's move apart and in the same box. The box rule already forbade them from matching, so the Anti-Knight rule adds nothing there. The eliminations that actually change the puzzle are the ones that cross a box border, and those are the ones to hunt for.
Solving Strategies
1. Fire the eliminations the moment you place a digit
The single biggest mistake in Anti-Knight is placing a digit, moving on, and coming back to the knight constraint later. By then the board has changed and you will not remember which cells still need clearing. Make it a reflex: place the digit, then immediately walk the eight L-shaped destinations and strike the digit from each.
2. Trace the L in a fixed order
Pick one traversal order and always use it — for example clockwise starting from "two up, one right". A consistent route is what stops you skipping a peer, and skipping a peer is how an Anti-Knight solve quietly goes wrong twenty moves before you notice.
3. Look for repeated digits in adjacent boxes
Anti-Knight bites hardest between boxes that touch. If a digit is already placed in a box, the knight rule forbids it from a specific handful of cells in the boxes above, below, left, and right — usually enough to reduce that digit to one or two candidates in the neighbouring box. Chasing a single digit across the three boxes of a band is the most productive habit in this variant.
4. Use the constraint to break ties, not to start
Anti-Knight rarely gives you an opening move on its own. Its value is in the middlegame, where a cell is down to two candidates and ordinary row-column-box logic has run out. Checking whether one of those two candidates has a knight peer already holding it resolves the cell instantly, and that is where most Anti-Knight progress actually comes from.
5. Keep the standard toolkit sharp
Naked singles, hidden singles, pointing pairs and everything else in our solving techniques library work unchanged here. The knight rule feeds them: it prunes candidate lists so that patterns which were not quite there become visible.
Mini Anti-Knight Sudoku (6x6)
Mini Anti-Knight applies the same rule to a 6x6 grid using the digits 1 through 6 with 2x3 boxes. The knight's move is identical — two cells one way, one cell across — but on a smaller board more of those moves fall off the edge. A cell in the middle four still has all eight peers; nearer the border the count drops away, and a corner cell has just two.
That thinner ring of peers is exactly what makes the Mini version the right place to start. You can hold every peer of a cell in your head at once, which turns the L-shaped scan from an exercise in bookkeeping into something you simply see. Mini Anti-Knight is free to play; the 9x9 version is part of the premium collection.
Tips for Beginners
- Say the move out loud at first. "Two up, one left." It sounds silly and it works — the L becomes automatic within a puzzle or two.
- Let the app show you. Selecting a cell in Sudoku - Brain Puzzles highlights all of its knight peers, so you can check your instinct against the board instead of trusting it.
- Diagonal neighbours are not knight peers. The cell touching yours corner-to-corner is unconstrained by this rule. Confusing Anti-Knight with Anti-King is the most common beginner error.
- Edges are friendlier than the middle. Cells near a border have fewer peers, so start your knight scans in the interior where the rule has the most to say.
- Watch the same digit across a band. Following one digit through three side-by-side boxes is where the knight rule pays out most reliably.
- Start on Mini Anti-Knight. The free 6x6 grid teaches the L-shaped scan on a board small enough to verify at a glance.
Anti-Knight is the quietest of the variants in this collection — nothing is drawn on the board, nothing is shaded, no clue sits in the margin. All the extra difficulty lives in a geometric relationship you have to carry in your head. Once the L becomes second nature, though, the rule stops feeling like extra work and starts feeling like extra reach: every digit you place ripples further across the grid than it ever could in Classic Sudoku.