Sudoku X Rules: How Diagonal Sudoku Changes the Game

The grid

Sudoku X uses the identical 9x9 grid as Classic Sudoku — 81 cells split into nine 3x3 boxes. The only visual difference is the two diagonal lines running through the board, which our version tints so you can spot them at a glance. Everything else about the layout, the clues, and the way you interact with the grid is unchanged from Classic.

The rules: standard, plus one

Every Classic Sudoku rule still applies in full:

• • Each row contains 1–9 exactly once — no repeats.
• • Each column contains 1–9 exactly once — no repeats.
• • Each 3x3 box contains 1–9 exactly once — no repeats.

Sudoku X adds a fourth requirement on top of those three:

• • Each main diagonal — top-left to bottom-right, and top-right to bottom-left — also contains 1–9 exactly once, no repeats.

All four rules hold simultaneously. A digit that's perfectly legal for its row, column, and box can still be wrong if it repeats a digit already sitting elsewhere on the same diagonal — that's the entire extra layer Sudoku X adds. The two diagonals cover 17 cells total (9 + 9, minus the center cell they share), so most of the grid is still governed only by the original three rules.

A worked example

Look at the highlighted X in the image above. Nine clue cells sit on the two diagonals: reading the top-left-to-bottom-right diagonal gives 8, 4, 6, 1, 9, and the top-right-to-bottom-left diagonal gives 3, 7, 6, 2, 5 (the 6 in the center is shared by both). Between them, every digit from 1 to 9 already appears exactly once across the X — which means, in a fully solved grid, no other cell on either diagonal could ever legally hold any of those nine digits again.

That's the practical value of the diagonal rule: once you've located every digit already placed somewhere on a diagonal, you've effectively pre-solved that entire line, the same way a nearly-full row tells you what's left to place. It's an extra check to run alongside your usual row, column, and box scanning — not a replacement for it. Our solving techniques guide covers the underlying scanning and elimination methods that still do most of the work in Sudoku X.

Why Sudoku X plays differently

The diagonal rule rewards a habit most Classic Sudoku players don't build by default: glancing along both diagonals whenever you place or consider a digit near the center of the board. A move that looks safe from the row/column/box perspective can still conflict with a digit two boxes away on the same diagonal, so cells near the diagonals often resolve faster than you'd expect once you start checking that fourth line out of habit. It's the same underlying logic as Classic Sudoku, just with one more constraint to fold into your scanning routine.

Difficulty levels

Sudoku X ships with slightly more starting clues than Classic Sudoku at each difficulty label, to offset the extra deduction the diagonal rule demands:

Common rule mix-ups

• • Only checking one diagonal — both diagonals are independent rules; satisfying one doesn't say anything about the other.
• • Forgetting cells off the diagonal are unaffected — the diagonal rule only constrains the 17 cells that sit on an X line; every other cell still follows only the row, column, and box rules.
• • Assuming the center cell has special rules — it doesn't get a different rule, it just belongs to two groups (both diagonals) at once, the same way a normal cell belongs to one row, one column, and one box.
• • Mixing up Sudoku X with Killer Sudoku — Sudoku X adds two more no-repeat lines; it doesn't add any sums or cages, so it's still pure placement logic like Classic.

Frequently asked questions