How it works
X-wing is the first chain technique most solvers meet. It looks at a single digit across the entire grid and uses a rectangular pattern to force eliminations.
The setup: a digit X has exactly two candidate cells in row A, exactly two candidate cells in row B, and those four cells happen to align across the same two columns. Whichever way the puzzle resolves, two of the four corners will be filled with X and the other two cannot be.
Because every column needs exactly one X, the columns are committed to having their X at one of the rectangle corners. Therefore X cannot appear elsewhere in those two columns. Cross it out.
When to look for it
Pick a digit. Find two rows where the digit has exactly two candidate cells, both on the same two columns. Symmetric logic works with rows and columns swapped.
Tips for spotting the pattern
- Track digits that have only two or three candidate cells per row. Those are X-wing fuel.
- Pivot the search: rows-then-columns is one direction, columns-then-rows is the other.
- X-wings often produce one elimination that triggers a single, which cascades the rest of the puzzle.
Common mistakes
- Forgetting that all four cells must be candidates for the same digit. Stray digits in the corners do not invalidate the pattern.
- Eliminating the wrong digit. X-wings only act on the digit they are built around.
- Confusing X-wing with naked pair. The X-wing spans rows and columns; the naked pair is within one unit.