Techniques › X-Wing
X-Wing
One digit, two rows, two columns. The first pattern that spans the whole grid.
Take a single digit. Find two rows in which it has exactly two possible cells, and where those cells sit in the same two columns. That is an X-Wing.
Whichever way it resolves, the digit occupies one column in the first row and the other column in the second — it cannot do otherwise without repeating in a column. So both columns are spoken for, and the digit can be cleared from every other cell in them.
This is the first technique that reasons across the whole board rather than inside one house, and for most players it is the real step up from intermediate to hard sudoku.
A worked example
Taken from a real puzzle in our bank, at the point where this was the cheapest move available. Pattern candidates are marked in amber; anything the step eliminates is struck through in red.
| 4 | 7 | 8 | 26 | 3 | 9 | 1 | 26 | 5 |
| 5 | 3 | 9 | 1 | 68 | 28 | 26 | 7 | 4 |
| 1 | 6 | 2 | 5 | 7 | 4 | 3 | 9 | 8 |
| 6 | 8 | 4 | 7 | 1 | 5 | 9 | 3 | 2 |
| 7 | 5 | 3 | 9 | 2 | 6 | 8 | 4 | 1 |
| 9 | 2 | 1 | 48 | 48 | 3 | 7 | 5 | 6 |
| 2 | 4 | 6 | 3 | 9 | 1 | 5 | 8 | 7 |
| 8 | 9 | 5 | 246 | 46 | 7 | 26 | 1 | 3 |
| 3 | 1 | 7 | 268 | 5 | 28 | 4 | 26 | 9 |
- NudgeDigit 6 lines up in a rectangular pattern across several lines. Nothing outside it can be 6.
- WhereX-Wing in row 1, row 9, column 4, column 8: r1c4, r1c8, r9c4, r9c8
- Do itRemove r8c4 ≠ 6
How to spot it
Pick a digit and mark every row where it has exactly two homes. Compare those rows in pairs: if two of them use the same column positions, you have one. Then do the same with columns.
Where you will meet it
X-Wing sits at tier 3 of our ladder, which means it first becomes necessary at Hard. Puzzles below that band never require it, though it may still appear as a shortcut.
Read next
- Swordfish An X-Wing with three rows and three columns instead of two.
- Jellyfish The four-row fish. Valid, beautiful, and almost never necessary.
- XY-Wing A pivot and two pincers. Whichever way the pivot falls, one pincer takes the digit.
- Simple Colouring Follow one digit's forced chain through the grid in two alternating colours.