Diagonal Sudoku (Sudoku X), Explained
Diagonal Sudoku, often called Sudoku X, keeps the familiar grid and adds one clever twist: the two main diagonals must also hold each value once. That small change opens up 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 also each hold every value once. So the cells on those diagonals answer to an extra constraint that ordinary cells do not.
Why the diagonals help you
Every extra rule is really an extra source of clues. 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 to make progress. Far from making the puzzle harder to start, the diagonals frequently give you your first footholds.
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. Check which values it still needs and where they can go.
- Use the diagonal to eliminate candidates. A value already on a diagonal cannot repeat anywhere else on that diagonal.
- Watch the center cell especially. It sits on both diagonals at once, so it is often heavily constrained.
- Keep switching between the normal units and the diagonals, since progress in one usually unlocks the other.
A tip for the diagonals
It helps to mentally highlight the two diagonals before you start, so you never forget they are there. A common mistake is to solve a diagonal Sudoku as if it were plain, place a value that breaks a diagonal, and only notice much later. Keeping the diagonals front of mind turns them from a trap into your best tool.
Where variants fit
Diagonal Sudoku is one of many variants that keep the core grid and add a rule, and trying them is a great way to deepen your logic once classic boards feel routine. 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.
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