X-Wing
Two matching lines give one digit exactly the same two possible homes. That rectangle removes the digit from the rest of those crossing lines.
What it is
Start with one digit, not with the whole board. If it can appear in exactly the same two columns in two different rows, those four possible homes make a rectangle. The digit must occupy one corner in each row, so both columns receive it from the rectangle. Any other note for that digit in those columns is impossible. The picture works the same way with rows and columns exchanged.
How to spot it
Choose one digit and scan its small notes. Keep only rows or columns where it has two homes. When two of them point to the same two crossing lines, pause: you have an X-Wing. Then clear the digit only from other cells on those crossing lines. You do not place a digit with the X-Wing itself.
Worked example
One question per step: choose 6, find two matching source lines, then clear the other notes on their cross-lines.
Step 1 of 4: Follow the 6
Ignore the other small notes. This lesson only asks where 6 can still go.
Why it matters
This is the first pattern that lets you connect distant parts of the grid with one calm argument. It is easier to learn when you ask one question at a time: which digit, which two matching lines, and which outside notes can now go.
Practise now
Solve a puzzle where this technique decides the game
Frequently asked questions
Why is it called an X-Wing?
Because the two valid placements form the diagonals of a rectangle, which trace an X. Only one diagonal is correct, but both cover the same two columns — which is what makes the elimination safe.
Do the four cells have to be in the same boxes?
No. The rectangle can span any two rows and any two columns anywhere on the board. Box boundaries are irrelevant to the pattern.
Where exactly can I remove the digit?
If you found the pattern in two rows, remove the digit from the two columns everywhere except the four rectangle cells. If you found it in two columns, remove it along the two rows instead.
What if a row has three candidate cells instead of two?
Then it is not an X-Wing. Exactly two cells per line are required. With three cells in three lines you may have a Swordfish, which is the next size up.
Put it to use
Where you will meet it
Hard grids are built around moves like this one. That is where it earns its keep.
Play HardStuck on a grid of your own?
Type the position into the solver and it walks the whole solution, naming the technique behind every step — including this one.
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