How to solve Sudoku X: making the diagonals work for you
5 min read ·
Sudoku X is classic Sudoku with one extra rule: the two long diagonals must also contain every digit from 1 to 9. That sounds like a harder puzzle, but in practice the diagonals are a gift. They give you two more lines to scan, which is why Sudoku X can start with fewer digits and still have exactly one solution.
Every technique from classic Sudoku still works here. This guide covers what changes when the diagonals join in, and how to use them from the very first move.
The rules
- Every row, column and 3×3 box contains the digits 1 to 9 exactly once.
- The two long diagonals, shaded on the grid, also contain 1 to 9 exactly once.
- Every puzzle on Puzzimo has exactly one solution; a solver checks it with the diagonal rule included.
New to Sudoku itself? The classic techniques (scanning, singles, pairs) are explained step by step in our Sudoku guide.
Think in eleven units, not nine
In classic Sudoku, each cell belongs to three units: its row, its column and its box. A cell on a diagonal belongs to four, and the centre cell to five. More units means more digits ruling candidates out, so diagonal cells usually have the fewest candidates on the board.
- Scan each diagonal like a row. Which digits are missing? Where on the diagonal can each one go?
- Hidden singles on a diagonal are common: a digit that fits only one diagonal cell must go there, even if that cell has other candidates.
- Count the diagonal cells' units. A diagonal cell sees its row, column, box and diagonal; collect the digits from all four before deciding it's open.
Start from the centre
The centre cell (row 5, column 5) sits on both diagonals, in the middle row, the middle column and the middle box. It sees more of the grid than any other cell, so it often resolves early, and once it does, both diagonals gain a digit at once.
After the centre, look at the corner boxes. Each holds three diagonal cells, so between their box, the diagonal and the crossing rows and columns, they tend to fill in quickly.
Where a diagonal crosses a box
Each diagonal runs through three boxes, three cells in each. That overlap works like the pointing pairs of classic Sudoku, with the diagonal as the line:
- If, inside a box, a digit can only go in that box's diagonal cells, it can't go anywhere else on that diagonal.
- If, on a diagonal, a digit can only go in the cells that cross one box, it can't go anywhere else in that box.
A solving routine
- Fill the centre cell if you can, then scan both diagonals for missing digits.
- Scan every box, row and column as in classic Sudoku.
- Check the corner boxes, where diagonal cells see the most.
- Write notes, and treat each diagonal as a unit when looking for pairs.
- Use the diagonal-and-box overlap to remove candidates, then go back to step 1.
Practise on the right level
Puzzimo's Sudoku X has three levels: Easy gives about 38 digits, Medium about 31 and Hard about 26. Notes, undo and hints are built in (a hint adds 30 seconds to a ranked time), and a wrong digit counts as a mistake; the third ends the round.
Frequently asked questions
Is Sudoku X harder than regular Sudoku?
Not necessarily. The diagonals add a rule, but they also add information, so puzzles can start with fewer digits. Many players find the diagonals make the opening easier once they start scanning them.
Which cell should I solve first in Sudoku X?
Try the centre cell. It belongs to five units (its row, column, box and both diagonals), so it usually has the fewest candidates on the board.
Do classic Sudoku techniques work in Sudoku X?
Yes, all of them. Singles, pairs and X-Wings work as usual, and the two diagonals count as extra units you can apply them to.