Diagonal Sudoku (Sudoku X), Explained
Diagonal sudoku, often called Sudoku X, keeps the familiar grid and adds one clever twist, so the two main diagonals must also hold each value once, opening a whole new layer of logic.
The one extra rule
Diagonal Sudoku follows every rule of classic Sudoku: each row, column, and three-by-three box must contain each value exactly once. On top of that, the two main diagonals, running corner to corner across the grid, must each hold every value once as well. So the cells sitting on those diagonals answer to an extra constraint that ordinary cells do not. Everything you already know about solving still applies; you are simply adding two more groups to keep an eye on, the diagonal from r1c1 down to r9c9 and the diagonal from r1c9 down to r9c1. Learn to watch those two lines and the rest of the board behaves exactly as you expect.
Why the diagonals help you
Every extra rule is really an extra source of clues, not just an extra burden. A cell on a diagonal now belongs to four groups instead of three: its row, its column, its box, and its diagonal. That means more values can be ruled out of it, which often makes diagonal cells the easiest places on the whole board to make progress. Far from making the puzzle harder to start, the diagonals frequently hand you your first footholds. Whenever you feel stuck in a plain region, glancing back at the two diagonals is one of the quickest ways to find a fresh forced placement and get the board moving again.
How to solve one
- Solve the classic way first, scanning rows, columns, and boxes for forced placements.
- Then treat each diagonal as its own unit, checking which values it still needs and where they can go.
- Use the diagonal to eliminate candidates, since a value already on it cannot repeat anywhere else along it.
- Watch the center cell especially, because it sits on both diagonals at once and is often heavily constrained.
- Keep switching between the normal units and the diagonals, as progress in one usually unlocks the other.
A worked example on the center
The cell r5c5 is the most powerful spot on a diagonal board, because it belongs to five groups at once: row 5, column 5, the center box, and both diagonals. Suppose the main diagonal from r1c1 to r9c9 already contains seven different values, and column 5 supplies one more between them. That can leave r5c5 with a single legal value even while nearby ordinary cells still have several. Placing it then ripples outward along both diagonals at once, often unlocking r3c3, r7c7, or the mirror cells on the anti-diagonal in the very next step. The center is almost always worth checking early and often.
A tip for the diagonals
It helps to mentally highlight the two diagonals before you start, so you never forget they are there. The most common mistake in a diagonal Sudoku is to solve it as if it were plain, place a value that happens to break a diagonal, and only notice the contradiction many moves later, after real damage is done. Keeping the diagonals front of mind turns them from a hidden trap into your best tool. Some solvers lightly mark the diagonal cells at the start, and others simply pause before each placement on one to ask whether that diagonal already holds the value they are about to write in.
How difficulty compares, and where variants fit
A diagonal Sudoku is not automatically harder than a classic one; it is different. The extra constraints give you more to track, but they also give you more to work with, and a well-made diagonal puzzle can start with fewer given cells precisely because the diagonals do so much of the heavy lifting. What changes is the texture of the solve, with more time spent on those two special lines and the crowded center. It is one of many variants that keep the core grid and add a single rule, and the underlying skill, reasoning by elimination across overlapping groups, is exactly the same one you build on any Sudoku, including the shape-based grids in Shapedoku. Learn it once and every variant becomes a friendly new challenge rather than a foreign one.
A quick history of Sudoku X
Sudoku itself is younger than most people assume, popularised in Japanese puzzle magazines in the 1980s before sweeping the world in the mid-2000s, and the diagonal variant followed naturally once solvers began hungering for something a little different. The X in Sudoku X simply refers to the two crossing diagonals, which form an X shape when you trace them across the grid. Its appeal is that it changes almost nothing about how the puzzle looks yet meaningfully changes how it solves, which is the mark of a well-designed variant. That balance is why it has stayed one of the most popular twists rather than fading as a novelty. If you enjoy how a single added rule reshapes the whole solving experience, Sudoku X is one of the friendliest places to begin exploring the wider family of variants.
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