An X-Wing is a candidate-elimination technique for a single digit. Its rectangle is visually memorable, but the rectangle alone proves nothing; the restriction inside the two base units creates the deduction.
What is an X-Wing in Sudoku?
An X-Wing occurs when one candidate appears in exactly the same two columns within two different rows, or in the same two rows within two columns. The candidate must occupy opposite corners of that rectangle, so it can be removed from other cells in the two shared columns or rows.
What does an X-Wing example look like?
Suppose candidate 7 appears exactly twice in row 2: columns 3 and 8. In row 6, candidate 7 also appears exactly twice: columns 3 and 8. Row 2 must place its 7 in one of those columns, and row 6 must use the other column.
Whichever diagonal pair becomes true, columns 3 and 8 already receive their required 7s from rows 2 and 6. Candidate 7 can therefore be removed from every other unresolved cell in columns 3 and 8.
How do you find an X-Wing?
- Choose one candidate digit.
- Scan rows where that digit appears in exactly two cells.
- Compare the column positions of those pairs.
- When two rows share the same two columns, verify all four corners.
- Remove the digit from other cells in the shared columns.
Then repeat the process with rows and columns reversed. A column-based X-Wing uses two columns as bases and removes candidates from the two matching rows.
Which candidates can an X-Wing eliminate?
Eliminate only from the cover units. In the row-based example, rows 2 and 6 are the base units and columns 3 and 8 are the cover units. Remove 7 from columns 3 and 8 outside the two base rows. Do not remove either candidate from the four corners.
The four corners do not need to be in the same boxes. What matters is that each base row has no other candidate positions for that digit and that the two pairs use identical columns.
What are common X-Wing mistakes?
- One base row has a third candidate: the two-corner guarantee disappears.
- The columns almost align: both pairs must use the same cover columns.
- Eliminating from base rows: remove candidates from cover units outside the corners.
- Mixing digits: all four corners concern the same candidate.
- Finding a rectangle of solved values: X-Wing is a candidate pattern, not a shape made from givens.
After an X-Wing elimination, update the affected cells and restart with singles. The useful result is often not an immediate placement but a newly exposed single or pair.
Free tool / Paper-ready
Want to test the rule on your own puzzle?
Take or upload a clear Sudoku photo, review every recognized given, and choose one hint or the full solution. The editable transcription matters: correct any faint or misread digit before asking the solver to reason from the grid.
Open the Sudoku Solver from Photo →About our game: Detective Sudoku applies one-per-row-and-column deduction to suspects, rooms and truthful clues. It is not classic Sudoku, and its real hints explain a clue-driven deduction instead of giving away the answer or solution.
Frequently asked questions about X-Wing Sudoku
Does an X-Wing always form a rectangle?
Its four candidate positions form rectangle corners, but the proof comes from exact candidate restrictions in the base units.
Can an X-Wing use columns instead of rows?
Yes. Two columns may restrict the candidate to the same two rows, producing eliminations in those rows.
Can X-Wing place a digit directly?
Usually it removes candidates. Those removals may then create a forced placement.
What should I learn after X-Wing?
Learn Swordfish for three-line fish, then consider XY-Wing and simple coloring.