What Is Sudoku X? Diagonal Sudoku Explained

If you've solved a few hundred classic sudoku and want the same familiar grid with a fresh twist, Sudoku X is the natural next step. It keeps everything you already know and adds just one rule — but that single rule changes how the whole puzzle solves. Here's what Sudoku X is, how it works, and why the diagonals matter.

The rules of Sudoku X

Sudoku X uses the same 9×9 grid as regular sudoku, and the same core rules: every row, every column, and every 3×3 box must contain the digits 1 through 9 with no repeats. The one addition is the 'X': both main diagonals — the one running top-left to bottom-right, and the one running top-right to bottom-left — must also each contain 1 through 9 with no repeats.

On our puzzles those two diagonals are shaded so you can spot them instantly. The shaded cells form an X across the grid, which is where the name comes from.

Why the diagonals change the puzzle

In classic sudoku, each cell belongs to three regions: a row, a column, and a box. In Sudoku X, the 17 cells that lie on a diagonal belong to a fourth region too. That extra membership means those cells are more constrained — and constraints are exactly what you use to solve.

The center cell is special: it sits on both diagonals at once, so it's shared between five regions. Because the diagonals carry information, the same number of starting clues tells you more than it would in a plain sudoku, and clever solving paths open up along the shaded lines that simply don't exist in the classic game.

A few solving tips

• Scan the two diagonals early — they're a fresh set of 'must contain 1–9' lines that classic sudoku doesn't have.
• Watch the center cell: it's on both diagonals plus a row, column, and box, so it's often pin-able from very little.
• When a diagonal has most of its digits filled, the missing ones are heavily constrained — a quick way to place tricky numbers.
• Otherwise, use the same techniques you already know: naked singles, hidden singles, and scanning rows, columns, and boxes.

Make your own printable Sudoku X

Our Sudoku X generator builds a fresh puzzle every time, with the diagonals shaded and a guaranteed unique solution. Pick easy, medium, or hard, play it right in your browser with a check-and-reveal helper, or download a clean printable PDF with the answer key on page two. It's free, works on paper in black and white, and needs no signup.