How it's different
Sudoku X uses the same 9x9 grid and 3x3 boxes as classic Sudoku, with one real addition: both diagonals - corner to corner, in an X shape - must also contain the numbers 1 through 9 exactly once. That's 17 cells carrying an extra rule beyond their row, column, and box. They're marked with a distinct border and tinted background, so you can see exactly which cells are part of the X without needing to click on anything first.
How to play
Controls are identical to classic Sudoku - the only difference is the extra diagonal rule, and the board marks which cells it applies to for you:
Cell First mode: click the cell you want to fill in first, then select the number you want to enter.
Number First mode: select the number you want to enter first, then click the cell you want to fill it into.
Memo mode: pencil in a memo of candidate numbers in a blank cell, instead of committing to one right away.
- Watch for the marked diagonal cells - a duplicate number anywhere on that same diagonal gets flagged red, even if it doesn't share a row, column, or box.
- Made a mistake? Undo steps back through your moves.
- Stuck? The hint button reveals one correct cell for you.
- Solve the board completely and it submits automatically.
Rules
Fill in the blanks with numbers from 1 to 9. Each row, column, 3x3 box, and both diagonals (marked on the board) must contain the numbers 1 through 9 exactly once.
Strategy tips
- Treat the marked diagonals like extra rows. Once a number appears anywhere on a diagonal, rule it out everywhere else on that same diagonal - the same way you would for a row or column.
- The center cell carries double duty. It sits on both diagonals at once, so it's often one of the most constrained cells on the board - worth checking early.
- Cross-reference diagonal cells against their box. A cell on the X is limited by its row, column, box, and diagonal all at once - four constraints instead of three often narrows it down fast.
- Start with the fullest rows, columns, and boxes, same as classic Sudoku - the diagonal rule adds to this, it doesn't replace it.
FAQ
How do I know which cells are part of the X?
They're marked with a colored border and a tinted background, visible at all times - no need to click a cell to see which ones carry the extra rule.
Do the diagonals replace the row/column/box rules?
No - they're an addition. Every cell still needs to satisfy its row, column, and box exactly like classic Sudoku; diagonal cells just have one more rule on top.
Is there only one solution to each puzzle?
Yes - and it's guaranteed to satisfy the diagonal rule too, not just rows/columns/boxes.
Can I play the same difficulty more than once in a day?
You can, just for fun - but only your first correct solve of the day counts toward today's leaderboard, and that difficulty locks afterward.
Does switching between numbers, letters, icons, and colors change the puzzle?
No - purely visual, the underlying logic (including the diagonal rule) never changes.