Everything you need to know about the knight constraint
Anti-Knight Sudoku is a variant of classic Sudoku that adds one additional constraint based on how the knight moves in chess. If you already know how to play standard Sudoku, learning Anti-Knight Sudoku takes only a few minutes. If you're new to Sudoku entirely, don't worry — we'll cover the basics here too.
A standard Sudoku puzzle is played on a 9×9 grid divided into nine 3×3 boxes. The goal is to fill every cell with a digit from 1 to 9 such that:
Some cells start pre-filled with digits (called "givens" or "clues"). Your job is to fill in the rest using logical deduction.
Anti-Knight Sudoku adds one more rule on top of the standard three:
In chess, a knight moves in an L-shape: two squares in one direction (horizontal or vertical), then one square perpendicular to that. From any given cell, there are up to 8 cells a knight's move away. If a cell contains the digit 5, none of those 8 knight-reachable cells can also contain 5.
Imagine a knight standing on a cell. It can "jump" to any of the following positions: 2 cells up and 1 cell left/right, 2 cells down and 1 cell left/right, 2 cells left and 1 cell up/down, or 2 cells right and 1 cell up/down. That's 8 possible destinations in the middle of the board, fewer near the edges and corners.
Start with the techniques you already know. Scan rows, columns, and boxes for digits that have only one possible location. Use the process of elimination to narrow down candidates for each empty cell.
After every placement, check the 8 cells a knight's move away and eliminate that digit from their candidate lists. This is where Anti-Knight Sudoku diverges from standard Sudoku — you must constantly track knight relationships.
The knight constraint is not just an obstacle. It actively helps you solve the puzzle. When a cell has multiple candidates in standard Sudoku, the knight constraint often eliminates some of them, revealing the correct answer. Experienced players learn to "see" knight conflicts at a glance.
If you're new to Anti-Knight Sudoku, start with our easy puzzles. They have more given digits and straightforward solution paths that let you learn the knight constraint without feeling overwhelmed. Work your way up through medium, hard, and finally expert difficulty.
Always use the notes feature to track candidate digits. This is essential at harder difficulties where you can't hold all the possibilities in your head. Remember: Anti-Knight Sudoku is a pure logic puzzle. Every puzzle has exactly one solution, and you should never need to guess.
Ready to try Anti-Knight Sudoku? Pick your difficulty and start solving: