Sudoku X: play diagonal sudoku online

Sudoku X, also called diagonal sudoku, adds one simple rule to the classic: both long diagonals must also contain the digits 1 to 9. Play Sudoku X online free here at four ranks.

Two extra houses sound minor, but they connect boxes 1, 5 and 9 and boxes 3, 5 and 7, and give you a fresh line to scan for singles.

The rules

• Normal sudoku rules apply: 1 to 9 once in every row, column and 3×3 box.
• The falling diagonal (top-left to bottom-right) contains each digit from 1 to 9 exactly once.
• The rising diagonal (bottom-left to top-right) contains each digit from 1 to 9 exactly once.

How to start

Treat each diagonal as a house

Scan a diagonal just as you would a row: for each digit, rule out the cells whose row, column or box already contains it.

The centre cell

The centre cell r5c5 lies on both diagonals, as well as its row, column and box. It sees more cells than any other, so it often resolves early.

Pointing along a diagonal

If, within box 1, a digit can only go on the diagonal cells, it can be removed from the rest of that diagonal in boxes 5 and 9. It is the same pointing idea as in classic sudoku, with a new kind of line.

Common mistakes

• Forgetting the diagonals while scanning.
• Checking only one diagonal. Both must hold 1 to 9.
• Thinking shorter diagonals count. Only the two long diagonals are houses.

How the ranks scale difficulty

Every puzzle type on Hoot’s Puzzles comes in four ranks, named after the stages of a mathematical idea. Lemma is the warm-up: about a dozen more digits are given than logic strictly needs, and singles plus the variant’s own rule will carry you through. Proposition needs a real idea or two, typically pointing and claiming. Theorem is near-minimal, so almost every clue is load-bearing and you may need pairs. Conjecture gives the fewest digits and sparser rule clues: expect to think hard.

Diagonal sudoku has no extra clue marks, so rank is all about the givens: fewer digits at the start and deeper logic at higher ranks.

There are 99,999 puzzles per rank, each with its own number, so you can replay one or share it. Every type also has a 30-puzzle collection that climbs from Lemma to Conjecture. Every puzzle is checked to have exactly one solution that you can reach by logic alone, with no guessing.

Keep exploring

• WindokuFour shaded windows are extra 3×3 houses. Also known as hyper sudoku.
• Anti-Knight SudokuNo digit repeats a chess knight’s move away.
• Classic SudokuRows, columns and boxes, nothing else. Pure logic at its cleanest.
• Solving techniquesSingles, pairs, cage arithmetic, line logic and Latin squares, all in one guide.
• All puzzle guidesEvery puzzle type on Hoot’s Puzzles, with rules and tips.