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What Is X-Sudoku? Diagonal Sudoku Rules, Explained

2026-06-22

If you've solved a few normal Sudoku and want a fresh twist, X-Sudoku (also called Diagonal Sudoku) is the easiest variant to pick up — it adds exactly one rule. You can try a board here while you read.

The extra rule

Classic Sudoku asks that every row, column, and 3×3 box contains 1–9 once. X-Sudoku adds one more constraint:

Both main diagonals (corner to corner) must also contain 1–9, with no repeats.

That's the whole difference. The "X" is the shape those two diagonals make across the grid — on ToolKoala's board they're lightly tinted so you can see them at a glance.

How it changes the solve

The diagonal rule is extra information, and good solvers learn to use it rather than fear it. The 17 cells on the diagonals get two overlapping constraints, so they often resolve earlier than in classic Sudoku — if you remember to check them. Beginners who ignore the diagonals find X-Sudoku harder; once you fold them into your scanning, it can actually be faster.

A few habits help:

  • When you place a number, check its diagonal too, not just the row, column, and box.
  • The center cell sits on both diagonals — it's heavily constrained, so it's often a good place to look for a single.

Is it harder than normal Sudoku?

Usually a little, because the added rule removes options you'd otherwise have to keep open. But it's a gentle step up — the same logic, just one more line to watch. If you're comfortable finding hidden singles in classic Sudoku, you're ready for X.

Try it

Open Diagonal Sudoku, pick a difficulty, and keep an eye on the two tinted diagonals. Prefer pen and paper? You can print a PDF pack of diagonal puzzles too.

FAQ

What is X-Sudoku? It's Sudoku with one extra rule: each of the two main diagonals must also contain 1–9, on top of the usual rows, columns, and 3×3 boxes.

Why is it called "X"? The two corner-to-corner diagonals form an X across the grid. Those are the cells with the extra constraint.

Is X-Sudoku harder? Slightly — the diagonal rule removes options. But it also gives you extra clues, so once you use the diagonals it's a smooth step up from classic Sudoku.

Does every X-Sudoku have one solution? Yes. Each puzzle is generated to have exactly one solution, diagonals included, so it's always solvable by logic.

— Milo 🐨