Build the row-based pattern
Choose a digit and scan the candidate positions row by row. Find a row where the digit appears in exactly two cells. Then find a second row where that digit also appears in exactly the same two columns. Those four cells form the X-Wing. In each row, one of the two cells must contain the digit, which forces the two columns to receive the digit within the four corners.
Because each column can contain the digit only once, candidates for that digit in other rows of those two columns can be eliminated. You still cannot determine which diagonal pair is correct from the X-Wing alone.
Use the column-based mirror
The pattern works in the opposite direction as well. If two columns each contain the candidate in exactly the same two rows, eliminate that candidate from other cells in those rows. State the base units and cover units clearly: the units with exactly two candidates are the bases; the intersecting lines where eliminations occur are the covers.
Verify every corner
An apparent rectangle is not enough. Each base row must contain exactly two candidates for the chosen digit, and their columns must match. Extra candidates in either base row break the strong restriction. Candidate highlighting is helpful for detection, but verify the count manually before removing anything. X-Wing errors are expensive because eliminations may be far from the visual center of the pattern.
- Choose one digit and display or inspect all of its candidates.
- Find a row with exactly two candidates for that digit.
- Find another row with exactly two candidates in the same columns.
- Confirm the four corners are distinct and all candidate data is current.
- Remove the digit from other cells in the two cover columns.
Know when to search for it
Do not begin every puzzle with an X-Wing hunt. Resolve singles, pointing interactions, and pairs first because they are easier to verify and often simplify the grid. Search for X-Wings when a difficult puzzle has stable full candidates and one digit appears repeatedly in two-position patterns across several lines.
Frequently asked questions
Must an X-Wing rectangle be a square?
No. The distances between the rows and columns do not matter. Only the matching candidate positions and unit constraints matter.
Does an X-Wing place one of its four candidates?
Not by itself. It eliminates the digit from other cells in the cover lines, which may then create a placement elsewhere.