Do the diagonals make the puzzle easier or harder?

The extra constraints usually give more deductions, but designers compensate with fewer givens, so difficulty stays balanced.

Does only the main diagonal count?

Both main diagonals count. Shorter broken diagonals are not constrained.

Where should I start a Sudoku X puzzle?

At the centre cell and along the two diagonals. The centre belongs to five units at once, so it is usually the easiest early placement, and the diagonals frequently hold hidden singles that a classic grid would not.

Sudoku X Rules

Classic Sudoku with two extra diagonal constraints forming an X across the grid.

Sudoku X adds two constraints to the classic puzzle: the two main diagonals must each also contain the digits 1–9 exactly once. The diagonals cross in the centre cell, tracing an X — hence the name.

The extra constraints give you more to work with and often make otherwise-hard grids tractable.

The rules

All classic rules apply: 1–9 once per row, column and box.
The main top-left to bottom-right diagonal contains 1–9 once.
The main top-right to bottom-left diagonal contains 1–9 once.
The centre cell belongs to both diagonals.

The diagonals

The top-left-to-bottom-right diagonal and the top-right-to-bottom-left diagonal each act like an additional unit. Treat them exactly as you would a row or column when scanning for singles and eliminations.

Sudoku X adds two units: both main diagonals must hold 1–9 — and the centre cell sits on both.

Using the diagonals

The centre cell is the most constrained on the board — it sits in a row, a column, a box and both diagonals, so five units bear on it. Solve outward from there. Each diagonal is a ninth and tenth unit for hidden-single and naked-subset scanning, and a digit placed on a diagonal eliminates it from that diagonal just as a row placement does.

Diagonals also create constraints that span boxes the classic rules cannot, so X-Wings and pairs that look impossible on a plain grid often become available. Always re-scan both diagonals after every placement.

Start at the heavily-constrained centre cell.
Scan each diagonal for hidden singles like any other unit.
Re-check both diagonals after placing any digit on them.

Frequently asked questions

Do the diagonals make the puzzle easier or harder?: The extra constraints usually give more deductions, but designers compensate with fewer givens, so difficulty stays balanced.
Does only the main diagonal count?: Both main diagonals count. Shorter broken diagonals are not constrained.
Where should I start a Sudoku X puzzle?: At the centre cell and along the two diagonals. The centre belongs to five units at once, so it is usually the easiest early placement, and the diagonals frequently hold hidden singles that a classic grid would not.

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