Variants
How Sudoku X differs from classic
For solvers who already play classic Sudoku — what changes mentally when you switch to Sudoku X (Diagonal Sudoku), and what stays exactly the same.
This is the article for solvers who already play classic Sudoku and want to know what changes if they try Sudoku X — the variant also called Diagonal Sudoku. The honest summary is: very little, and what changes is mostly welcome.
What stays the same
Everything you already know about Sudoku still works. The 9×9 grid is the same. The digits 1 through 9 are the same. The row, column, and 3×3 box uniqueness rules are inherited verbatim. Pencil marks work the same way. The keypad and the keyboard inputs work the same way. The technique vocabulary — naked singles, hidden singles, naked pairs, hidden pairs, locked candidates, X-wings, Y-wings, all of it — works identically.
This last point is the big one. A solver coming to Sudoku X from classic doesn't have to learn a new technique catalogue. Every move you already make on a classic Sudoku fires on a Sudoku X exactly the same way. You don't lose any of the moves you've learned.
What's new
You gain two units. One diagonal — top-left corner to bottom-right corner — has to contain each digit from 1 to 9 exactly once, like a row. The other — top-right corner to bottom-left corner — has to do the same. The two diagonals cross at the centre cell, which therefore belongs to both.
That's it. Every classic technique now has two more units to fire on. A naked single in a row was three constraints; a naked single on a diagonal cell is four. A hidden single on the diagonal works exactly like a hidden single in a row, just with the diagonal as the unit. Locked candidates fire on the diagonal-meets-box intersection the same way they fire on a row-meets-box intersection.
The one move that's genuinely new is the forced diagonal cell, where row, column, box, and diagonal all together force a digit no three of them would have. That's the only deduction shape that has no equivalent in classic Sudoku, and even it isn't a new technique so much as a four-constraint version of an old one.
What changes mentally
The biggest adjustment is the scan pattern. In classic Sudoku, an experienced solver's eye sweeps three units when checking a cell: row, then column, then box. The sweep is muscle memory. In Sudoku X, cells on the diagonal need a fourth sweep — and your eye isn't trained for it.
For the first few puzzles, this means consciously remembering to include the diagonals. After three or four puzzles, the diagonal becomes part of the automatic sweep, and the conscious effort drops away. Most solvers report the transition happening around puzzle five.
The other adjustment is how much the diagonal helps. Most experienced classic solvers expect the diagonal to be a tiebreaker — the rule that helps in edge cases. In practice, the diagonal is more central than that. At medium and above, a meaningful fraction of the harder deductions go through the diagonal. The variant pulls its weight as a variant, not just as a flavour.
What gets easier, and what gets harder
Easy puzzles get a touch easier. The extra constraint gives you more places to spot hidden singles, which means fewer cells where you have to think.
Medium and hard puzzles stay about the same. The extra unit adds constraints, but the construction targets a similar overall difficulty, so the puzzle authors give you slightly fewer starting digits to compensate. The total amount of work is similar.
Expert and master puzzles get a little stranger. Some deductions that would have required a chain or fish in classic Sudoku resolve via a short diagonal-aware move in Sudoku X. Other deductions go the other way — the variant supports its own chain shapes that don't exist in classic. The net feel is that the puzzles use different deduction shapes than classic, not harder or easier shapes.
How to start
Don't start at expert. The temptation, for a strong classic solver, is to assume Sudoku X is "classic plus a small thing" and jump in at the tier you usually play. The diagonal-scan habit takes a few puzzles to form, and the expert tier is unforgiving of solvers who haven't built it yet. Start at easy or medium, get the scan into your eye over a few puzzles, then move up. The transition from classic-expert to Sudoku-X-expert takes about ten puzzles to feel natural.
Related reading
Rules & basics
The rules of Sudoku X
Sudoku X — also called Diagonal Sudoku — is classic Sudoku with one extra rule: both diagonals must also contain each digit 1–9 exactly once.
4 min read
Variants
Meet Sudoku X
An introduction to Sudoku X (a.k.a. Diagonal Sudoku) — what the diagonal rule adds, why classic solvers tend to enjoy it, and how to know if it's for you.
3 min read
Techniques
When the diagonal helps
A practical tactic for Sudoku X (Diagonal Sudoku): scan the diagonal when a row-column-box pass stalls. Worked examples of where the extra unit pays off.
4 min read
Glossary terms
- Sudoku X— A Sudoku variant where both diagonals must also contain each digit 1-9 exactly once. Also known as Diagonal Sudoku. Classic Sudoku plus two new units.
- Diagonal Sudoku— An alternate name for Sudoku X — the 9×9 Sudoku variant where both diagonals carry the same digit-uniqueness rule as rows, columns, and boxes.
- Diagonal constraint— The extra rule that defines Sudoku X (Diagonal Sudoku): each of the two diagonals must contain every digit 1-9 exactly once, just like a row or column.
- Forced cell on the diagonal— A Sudoku X (Diagonal Sudoku) deduction: a cell on the diagonal whose digit is forced by row, column, box, and diagonal together — no three of them suffice.
- Unit— Collective name for a row, column, or 3×3 box — the three groupings Sudoku's no-repeats rule applies to. Every cell sits in exactly three: its row, column, and box.