A short history of Sudoku X

Sudoku X — or Diagonal Sudoku, which is the older and more descriptive of the two names — is younger than classic Sudoku but not by much. Both are post-1979 puzzle designs that took their modern shapes during the brief, intense window in which the genre was finding itself.

Where it came from

The classic Sudoku grid as we now know it — the 9×9 layout with 3×3 boxes and the digits 1 to 9 — was published in 1979 by Howard Garns under the name Number Place in a Dell puzzle magazine. Diagonal variants appeared not long after. Through the 1980s and early 1990s, a number of puzzle constructors experimented with extra constraints on top of the basic number-placement frame: extra regions, killer-style cages, irregular box shapes, and — most simply — uniqueness on the two diagonals.

The diagonal variant was one of the cleanest extensions. It needed no new visual vocabulary, no new shaded regions or arithmetic, no change to the grid size. Just one extra rule, easy to state, easy to remember, easy to teach. By the time Sudoku as a genre exploded in popularity in 2004 and 2005 — when The Times of London began publishing it and the puzzle went global in the space of a few months — Diagonal Sudoku was already a known variant. It got pulled along in the wake of the boom rather than launched by it.

How it spread

Most of the early 2000s Sudoku spread happened through newspapers, books, and the first generation of online puzzle sites. The classic 9×9 was the default everywhere. But major outlets often ran a variant in the back of the puzzle pages — a Killer one day, a Diagonal Sudoku another, a Samurai (five overlapping 9×9 grids) on Sundays. Diagonal Sudoku tended to be the variant of choice for solvers who wanted slightly more puzzle than classic but weren't ready for the bigger cognitive jump that Killer or Samurai required.

The name Sudoku X came later and is the one that stuck in most English-language puzzle communities. Diagonal Sudoku — more literal — remained common in other languages and in scholarly contexts where naming things after their constraint set is preferred over naming them after their visual silhouette. Both names refer to the same puzzle.

The quietly steady following

Sudoku X never had a viral moment. It doesn't have the cultural cachet of the New York Times's Spelling Bee or the meme-volume of Wordle. What it has, instead, is a small and durable corner of the puzzle community — solvers who play it weekly, sometimes daily, often as a deliberate alternative to classic on days when classic feels too familiar. Puzzle constructors continue to publish Sudoku X regularly, and it appears in most major puzzle compendia.

A reasonable fraction of the people who eventually come to prefer Sudoku X to classic discover it the same way: they finish a classic Sudoku, feel mildly underwhelmed, try a Sudoku X out of curiosity, and find that the diagonal rule provides exactly the small extra hit of challenge their classic puzzles had stopped delivering. The variant doesn't try to convert anyone; it just waits.

Where it sits now

In 2026, Sudoku X is one of the three or four most commonly published Sudoku variants, alongside Killer Sudoku, Hyper Sudoku, and (depending on the publisher) irregular Sudoku. It's a mid-tier favourite — not a niche curiosity, not a mainstream phenomenon, but a steady fixture. Most online Sudoku platforms support it.

The variant's place is mostly settled. There isn't an ongoing argument about what the rules should be (unlike, say, the early debates over whether Killer Sudoku should allow repeated digits inside a cage — the answer settled to "no"). There aren't competing schools of construction. The puzzle is exactly what it has been since the early 1990s, and the people who like it like it for that.

If you've never tried one, the easy tier of Sudoku X is the gentlest possible entry point. Two diagonals, ten minutes, and a small chance you'll find a new puzzle to fold into your day.