X-Wing
A candidate forming a rectangle across two rows and two columns lets you eliminate it from the rest of those columns (or rows).
Candidate 5 appears in row 2 only at columns 3 and 7 — and in row 7 only at those same two columns.
How to spot it
- 1Find two rows where a candidate appears in only two cells each.
- 2Check those cells share the same two columns, forming a rectangle.
- 3Remove the candidate from the rest of both columns.
When to use
The gateway advanced technique — the first fish worth learning.
When not to
Don't force it: if the four cells aren't a clean rectangle, it isn't an X-Wing.
Common mistake
Misreading three-cell rows as an X-Wing. Each of the two rows must have exactly two positions.
Worked Example
Suppose candidate 5 appears in row 2 only at columns 3 and 7 — and in row 7 only at those same two columns. That's the rectangle: four cells, two rows, two columns.
Row 2 needs a 5 in column 3 or column 7. Row 7 needs a 5 in column 3 or column 7 too. Either row 2 supplies column 3's five and row 7 supplies column 7's — or the other way around. Whichever it is, both columns end up getting their 5 from these two rows. That means no other cell in column 3 or column 7 can be a 5, so every other 5 candidate in those two columns can be eliminated. That's the X-Wing.
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