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Variant · Two extra units

Diagonal Sudoku — the X factor

Both diagonals must hold 1–9 as well. Two extra units to exploit — if you remember to look at them.

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Tap a cell, then a number

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Classic board for unit-scanning practice — diagonal awareness builds directly on this skill. Native Sudoku X mode is on our roadmap.

Sudoku X rules

  1. Classic rules apply: every row, column and 3×3 box contains 1–9 once.
  2. Main diagonal: the nine cells from R1C1 to R9C9 contain 1–9 once.
  3. Anti-diagonal: the nine cells from R1C9 to R9C1 contain 1–9 once.

The center cell R5C5 belongs to both diagonals — it is the most constrained cell on the board.

The diagonal-scan strategy

Open on the diagonals. Most players scan rows first out of habit. In Sudoku X, the diagonals are your edge: each one is a unit your opponent forgets. Count which digits are missing from both diagonals before touching anything else.

Exploit double-constrained cells. Diagonal cells answer to three units instead of two (four for R5C5). Candidates there collapse fast — filling diagonal cells early creates a cascade of classic Hidden Singles.

Watch the wings. The cells where diagonals meet box edges (R3C3, R3C7, R7C3, R7C7) often anchor pointing patterns that clear entire rows. If you know Pointing Pair, you already own this skill.

Why Sudoku X is secretly easier

More units = more eliminations = fewer givens needed. A classic board might start with 30 clues; a fair Sudoku X often starts with 22–26 and still resolves cleanly. If classic Medium feels slow, Sudoku X is the rare variant that can feel faster — once the diagonals become reflex.

Keep exploring

All Sudoku variants · Jigsaw Sudoku · Samurai Sudoku · Killer Sudoku rules · Strategy ladder

FAQ

What is Diagonal Sudoku?
Diagonal Sudoku — usually called Sudoku X — is classic 9×9 Sudoku with two extra constraints: the main diagonal (top-left to bottom-right) and the anti-diagonal (top-right to bottom-left) must each also contain the digits 1–9 exactly once.
Why is it called Sudoku X?
Because the two shaded diagonals form an X across the grid. The name distinguishes it from other diagonal variants like Argyle Sudoku, which uses many shorter diagonals.
Is Sudoku X harder than classic Sudoku?
Usually a little easier at the same clue count — two extra units mean two extra sources of eliminations, so boards need fewer givens to stay unique. Beginners find it harder only because they forget to use the diagonals.
What is the key strategy for Diagonal Sudoku?
Treat the diagonals as a tenth and eleventh box. Scan them exactly like rows: any digit missing from a diagonal often has a forced home, and cells on the diagonals carry double constraint — resolve them early.
Do X-Wing and other classic techniques still work?
Yes — every classic technique applies unchanged, plus diagonal-aware versions. A Hidden Single can live on a diagonal, and a diagonal pointing pattern can clear a row. Advanced players also use 'diagonal X-Wing' where one base line is a diagonal.

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