Lesson 9 of 9
X-Wing
Four squares in a rectangle that force a digit into two columns, clearing both of them elsewhere.
What this grid does
- Starting digits
- 23 of 81
- Deductions to solve
- 60
- Uses of X-Wing
- 1
- Digits placed before it is needed
- 23
- First needed at
- deduction 25, in column 3 (R3C3, R6C3, R3C4 and R6C4, on the 6)
- Also calls for
- last free cell, cross-hatching, naked single, hidden single and pointing pairs
Measured on this grid by the same solver the hint button uses — how these puzzles are chosen.
Why this one is worth drilling
Only about 1 puzzle in 200 needs it. That is the whole reason this lesson exists: you cannot learn a deduction a random grid almost never asks you for, so it has to be set deliberately. That figure is measured rather than guessed: 0.5% of solvable 22-clue grids over 4,000 digs — about 1 in 200.
An X-Wing position looks exactly like a grid that needs a guess, which is why so many solvers guess there. Every unit has been worked, nothing is forced, and the deduction that breaks it spans four squares that may sit far apart. It is the one technique here whose absence genuinely stops a solve dead.
Where it sits in the nine
Last, because everything else has to be exhausted before it is even worth looking for, and because it is the only technique here that spans two units of the same kind rather than a box and a line. It is also the natural end of the path: past this point the techniques get sharply rarer and more intricate, while this one still turns up in real puzzles often enough to be worth the drill.
Working it
Do not look for it until everything else is exhausted — on a grid that still has singles in it you will waste the search. When the grid is genuinely stuck, take one digit at a time and list the rows where it has exactly two possible squares. Only then compare columns. Picking the digit first, and insisting on exactly two, is what turns an open-ended hunt into a short checklist. Rows before columns, and only ever one digit at a time. The search is symmetric — an X-Wing found from the rows could equally have been found from the columns — so checking both doubles the work for nothing. Settle on one direction, finish all nine digits, and only then switch. Most solvers who say they "can never find X-Wings" are searching both directions at once and losing track of which digit they were on.
The full explanation of x-wing covers the reasoning with diagrams.
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