X-Sudoku adds both main diagonals to the classic rules. Play free 4×4 mini and 9×9 boards from beginner to master difficulty, right in the browser.
Sudoku with a twist — the two diagonals must also contain every number. Six skill levels from Mini Starter to Master.
For ages 10 and up · No sign-up · Instant play
Standard Sudoku Rules
The X Rule (What Makes It Special)
Plenty of clues — solve with straightforward logic.
Requires basic elimination and diagonal awareness.
Multi-step logic. The diagonals become critical.
Near-minimal clues. Demands deep diagonal reasoning.
Fewest clues possible. Only for elite solvers.
1. Start with the diagonals
The two diagonal lines give extra constraints — shaded cells often have fewer candidates than non-diagonal cells.
2. Corner cells are special
The four corner cells appear on a diagonal AND a box AND a row AND a column — the most constrained cells on the board.
3. Center cell = all constraints
In 9×9, the center cell (row 5, col 5) sits on both diagonals — it's usually one of the easiest cells to solve.
X-Sudoku keeps classic row, column, and box rules, then adds both main diagonals. That extra constraint rewards careful candidate tracking and pattern spotting — the same logic habits used in planning and debugging.
You practise constraint satisfaction: every placement must satisfy more rules at once. Working memory holds candidates while you test hypotheses — a cousin of the skills used in coding, scheduling, and formal reasoning quizzes.
Unlike speed arcade games, Sudoku rewards calm systematic search. Timed personal bests still matter, but careless fills cost more because diagonals amplify mistakes.
Cells on the diagonals belong to four units instead of three. Early placements there prune the board faster — or lock you into a contradiction if you guess. Strong players seed the X first.
Notes mode is not cheating; it externalises working memory so you can focus on logic rather than remembering every candidate.
Scan for naked singles and locked candidates before you branch. Guessing early creates long undo chains.
Use the same difficulty ladder (easy → expert) so timing comparisons stay fair. Jumping difficulties muddies your personal bests.
Short daily boards beat rare marathon sessions for pattern fluency.