Find a digit that appears as a candidate in exactly two cells of one row, and exactly two cells of another row — with both pairs standing in the same two columns. The four cells draw a rectangle, and the digit must land on one of its diagonals. Either way, both marked columns receive the digit inside the rectangle, so every other candidate for it in those two columns is gone.
Digit 7 locked in rows 2 and 6 on columns 3 and 7 — the red 7s in those columns cannot survive.
Say digit 7 sits, as a candidate, only in r2c3 and r2c7 of row 2, and only in r6c3 and r6c7 of row 6. The four cells share columns 3 and 7 — a textbook X-Wing. Whichever diagonal the 7s take (r2c3+r6c7 or r2c7+r6c3), columns 3 and 7 each receive exactly one 7 inside the rectangle. So the 7 pencilled in r4c3, r8c3, r1c7 or r9c7 can be erased on the spot.
If a defining row holds the candidate in a third cell, the pattern collapses — that is not an X-Wing but a candidate for a finned variant, which has stricter elimination rules. Count twice, erase once.
These grids come from rated games on this site — our solver confirmed that x-wing was part of the shortest solving path for each one.
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