How to Play X-Sudoku (Diagonal Sudoku): Rules & Guide

X-Sudoku (also called Diagonal Sudoku) adds one elegant twist to classic Sudoku: both main diagonals must also contain the digits 1–9 exactly once. The two diagonals form an “X” shape across the grid, giving this variant its name. It’s a small change that creates surprisingly different solving dynamics.

The Rules of X-Sudoku

Rule 1 — Classic rules: Every row, column, and 3×3 box must contain the digits 1 through 9 exactly once. (Same as standard Sudoku.)
Rule 2 — Diagonal rule: Both main diagonals (top-left to bottom-right, and top-right to bottom-left) must also contain the digits 1 through 9 exactly once.

That is the only extra rule, but it changes the rhythm of solving. Every diagonal cell has four groups to check instead of three (row, column, box, and diagonal), so diagonal placements often become available earlier than they would in a classic Sudoku.

Understanding the Diagonals

The two diagonals each pass through exactly 9 cells:

  • Main diagonal (top-left → bottom-right): Row 1 Col 1, Row 2 Col 2, Row 3 Col 3, ... Row 9 Col 9
  • Anti-diagonal (top-right → bottom-left): Row 1 Col 9, Row 2 Col 8, Row 3 Col 7, ... Row 9 Col 1

The center cell (Row 5, Col 5) belongs to both diagonals, giving it extra constraints. This makes the center cell particularly useful for elimination.

💡 Key insight

Cells on a diagonal have four constraints (row + column + box + diagonal). Cells not on either diagonal have only three (row + column + box), just like classic Sudoku. This means diagonal cells are often easier to solve because they have more constraints!

Visual: The two diagonals in X-Sudoku
5
3
8
1
4
7
2
9
6
1

Light orange cells form the main diagonal (top-left → bottom-right). Darker orange cells form the anti-diagonal (top-right → bottom-left). The yellow center cell belongs to both diagonals — it has 5 constraints (row + column + box + both diagonals).

How the Diagonal Changes Your Strategy

1. Diagonal Elimination

When a digit is placed on a diagonal, it eliminates that digit from all other cells on the same diagonal. Think of each diagonal as an additional “row” or “column” that you need to check.

For example, if you place a 7 on the main diagonal at Row 3 Col 3, then no other cell on the main diagonal can contain 7. This eliminates 7 from Row 1 Col 1, Row 2 Col 2, Row 4 Col 4, and so on.

2. Diagonal Hidden Singles

Just like a row or column can have a Hidden Single, a diagonal can too. If digit 5 can only go in one cell on the main diagonal (because all other diagonal cells already see a 5 from their row, column, or box), then that cell must be 5.

Beginner Technique Diagonal Hidden Singles are the most common way the diagonal constraint helps you solve cells.

3. Diagonal-Box Interaction

Each diagonal passes through exactly three 3×3 boxes. When a digit on the diagonal is constrained within a box, it can create powerful elimination patterns similar to Pointing Pairs in classic Sudoku.

Step-by-Step Solving Strategy

1 Start with classic techniques

Begin exactly as you would in a normal Sudoku: scan for missing digits, fill obvious singles, and note the most constrained rows, columns, and boxes. X-Sudoku rewards solid fundamentals more than flashy pattern-hunting.

2 Check diagonals for each digit

After your first classic scan, switch perspective and test the diagonals digit by digit. Ask: “Where can 1 still go on this diagonal? Where can 2 go?” This is often where the variant stops feeling abstract and starts producing real placements.

3 Use diagonal elimination for pencil marks

When filling in pencil marks (candidates), remember to include the diagonal constraint. A cell on the diagonal has an extra group to check, which often means fewer candidates.

4 Combine all four constraints

The most powerful deductions come from combining all four constraints. When a cell is on a diagonal and at the intersection of a nearly-complete row and box, the diagonal constraint can be the final piece that determines the answer.

A Practical Scanning Routine for X-Sudoku

If you want X-Sudoku to feel manageable instead of chaotic, use the same scan order every time:

  1. Do one normal Sudoku pass first — rows, columns, boxes.
  2. Then scan the main diagonal digit by digit.
  3. Then scan the anti-diagonal digit by digit.
  4. Re-check the center box and the four corner boxes because diagonal pressure often changes them first.

This routine sounds simple, but it prevents the most common X-Sudoku problem: remembering that the diagonal rule exists only when you are already stuck.

Common X-Sudoku Mistakes

! Remembering the diagonal rule too late

Many players solve half the board like a normal Sudoku and only then start checking the diagonals. That wastes one of the variant’s biggest advantages: extra information early in the solve.

! Over-focusing on diagonals and ignoring ordinary singles

X-Sudoku is still built on classic Sudoku logic. If a row or box is almost solved, take the easy placement. Do not ignore simple progress because you are hunting for a fancy diagonal pattern.

! Forgetting that the center cell is special

Row 5, Column 5 belongs to both diagonals, plus its row, column, and box. When the puzzle feels stuck, the center and nearby diagonal cells are often worth re-checking first.

X-Sudoku Difficulty Levels

At Sudoku School, X-Sudoku puzzles come in five difficulty levels:

  • Easy: Many given digits. Diagonal Hidden Singles are straightforward.
  • Normal: Fewer givens. Requires systematic diagonal scanning.
  • Hard: Requires combining diagonal constraints with intermediate classic techniques.
  • Expert: Advanced techniques needed. Diagonal-box interactions become critical.
  • Master: The ultimate X-Sudoku challenge. Requires deep diagonal-aware reasoning.

Play X-Sudoku now and choose your difficulty!

When to Move Beyond the Basics

You are ready for advanced X-Sudoku study when the puzzle stops opening through simple diagonal singles and starts creating pressure patterns instead:

  • A diagonal digit has only two or three legal cells left, but none of them is immediately placeable.
  • The center cell or a corner diagonal cell keeps reappearing in your candidate checks, which means several houses are colliding there.
  • A box becomes interesting only after you include one diagonal, even though the classic row-column picture still looks ordinary.
  • You start seeing the same candidate echoed across diagonal-linked rows or columns, which is often the first sign of diagonal pointing or a more advanced pattern.

When that happens, move on to Advanced X-Sudoku Techniques. You do not need to memorise every advanced idea at once: learn diagonal hidden singles and diagonal pointing first, then add diagonal X-Wing and combined-constraint work later.

Frequently Asked Questions

Is X-Sudoku harder than regular Sudoku?

The extra diagonal constraint gives you more information, which can make some deductions easier. However, X-Sudoku puzzles are often constructed to require diagonal-aware thinking, which adds a layer of complexity. Most players find it a fun middle ground between classic and Killer Sudoku.

Why is it called X-Sudoku?

Because the two diagonals form an “X” shape across the grid. It’s also commonly called “Diagonal Sudoku” or “Sudoku X.”

Do all cells have diagonal constraints?

No. Only cells on the two main diagonals (18 cells total, with the center cell shared) have the extra diagonal constraint. The other 63 cells follow only classic Sudoku rules.

Can I use classic Sudoku techniques in X-Sudoku?

Absolutely! All classic techniques (Naked Singles, Hidden Singles, Naked Pairs, Pointing Pairs, X-Wing, etc.) work in X-Sudoku. The diagonal rule is an addition to the classic rules, not a replacement.

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