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.
Sudoku X is what happens when you take a classic Sudoku and ask it to behave on the diagonals too. Same 9×9 grid, same digits, same rows and columns and boxes — plus, now, two more units to keep track of. The two diagonals each have to contain every digit from 1 to 9 exactly once, just like a row. You'll also see it called Diagonal Sudoku, which is the more literal name and the one that searches well in most languages.
It's a small change. It's also, for most people who play classic Sudoku regularly, a surprisingly welcome one.
What the diagonals do for you
If classic Sudoku is a puzzle where you scan three units to figure out where a digit goes, Sudoku X is a puzzle where you scan four. The extra unit doesn't make easy puzzles harder — at the easy tier, the diagonals mostly just give you more places to spot a hidden single. What changes is the texture of the harder tiers. When a row-column-box scan stalls, the diagonal often breaks the stall.
The puzzles also tend to feel a touch more interconnected. In classic Sudoku, the digits in one corner of the board are mostly independent of the digits in the opposite corner — they share rows and columns and boxes only at the edges of the grid. In Sudoku X, the two corners share a diagonal, so a placement in the top-left can constrain the bottom-right directly. The grid feels more like a single thing and less like nine boxes that happen to be next to each other.
Is it for you?
Most people who enjoy classic Sudoku enjoy Sudoku X. The skills transfer cleanly. Scanning works the same way. Pencil marks work the same way. The technique vocabulary — naked singles, hidden singles, pairs, locked candidates — works the same way; the diagonals just give those techniques more units to fire on.
The ones who don't take to it tend to fall into two camps. There are solvers who like classic Sudoku precisely because it's a single, fixed rule set, and any addition feels like noise. And there are speed-solvers, who time themselves on classic puzzles and find the diagonal scan adds a step they don't want to learn. Neither is wrong; they're preferences. If you've ever finished a classic puzzle and wished it had been slightly more puzzle, Sudoku X is probably worth a try.
A small experiment
The honest test is to play one. Our easy Sudoku X puzzles take about the same time as easy classic puzzles, and the diagonal cells are marked with a dotted ring on the board so you don't have to remember where the X lives. If after one puzzle the diagonal rule feels like a welcome friend, you've found something to fold into your daily rotation. If it feels like extra homework, you've lost ten minutes and you know.
The variant has been around since the 1990s — older than most of the post-2005 Sudoku boom would suggest — and a small but loyal corner of the puzzle community has been quietly preferring it to classic for years. It's one of those puzzles where the people who play it think more people should play it. Try the first one and see which camp you're in.
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
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.
4 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.
- Diagonal pair— A naked pair where both cells lie on the same diagonal of a Sudoku X. Eliminates the two pair digits from every other cell on that diagonal.