An X-Wing occurs when one digit has exactly two candidate positions in each of two rows, and those positions share the same two columns. That digit can then be removed from other squares in those columns. The deduction works for one chosen digit, not for any rectangle of four empty cells.
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Fill each row, column, and 3 × 3 box with the digits 1–9 without repeats. Bold digits are fixed clues. Select a cell, then a number. Conflicts highlight duplicates; the absence of a conflict does not prove an entry is correct. Hint fills one cell with its solution value. Erase clears the selected entry; Undo restores the previous move. New puzzles have a unique solution.
Start with a normal 9 × 9 Sudoku. For the digit you are studying, rule out squares whose row, column, or box already contains that digit. The remaining positions are candidates. Your candidate information must be complete for the two rows used in the X-Wing; overlooking a third possibility breaks the proof.
Suppose you are considering 5. In row 2, its only candidates are column 3 and column 8. In row 7, its only candidates are also column 3 and column 8. Those four squares form the corners of a rectangle. The row numbers do not need to be adjacent, and the columns do not need to lie in the same box.
If row 2 takes its 5 in column 3, row 7 cannot also use column 3. Row 7 must therefore take column 8. If row 2 instead takes column 8, row 7 must take column 3. Either way, columns 3 and 8 receive their 5s from these two rows.
That leaves no room for another 5 in either column outside rows 2 and 7. You can eliminate 5 as a candidate from those other positions. The pattern does not tell you which diagonal arrangement is correct, so do not fill all four corners or choose a diagonal merely because it looks symmetrical.
The four corner candidates stay unresolved unless another clue decides between the two arrangements. X-Wing creates eliminations elsewhere, and those eliminations may lead to a single candidate in another square. The useful result might be outside the rectangle rather than a number you can immediately place at one of its corners.
Only the chosen digit is affected by this reasoning. If the pattern concerns 5, you cannot remove 6 or 8 from another square simply because that square lies in one of the columns. Record the digit and the two defining rows so you can explain each elimination if a later contradiction appears.
You can reverse rows and columns. If two columns each have the same two rows as their only candidate locations for one digit, eliminate that digit elsewhere in those rows. The proof is identical: either arrangement uses both required rows, leaving no place for another copy in them.
A third candidate in either defining row invalidates the row-based example. So does counting only candidates in one box while ignoring the rest of the row. Four corners alone are insufficient. Recheck the entire defining units, including squares far from the rectangle, before removing a candidate. A diagram that omits relevant positions can make a false X-Wing look convincing.
The playable board above lets you practice Sudoku, but it does not guarantee that its current puzzle requires an X-Wing. Try singles and simpler box-line deductions first. If those resolve the puzzle, there is no reason to force a more complicated pattern into the explanation.
On a phone, keep the two rows and columns clear while moving around the board. Make a complete candidate record for the chosen digit before applying the deduction. A normal hint supplies assistance with a cell; it does not explain this rectangle. Trace the four corner candidates and the outside eliminations yourself to understand why the X-Wing works.
No. It eliminates one digit outside the rectangle in the linked columns or rows.
Not for the simple row-based X-Wing described here.
Yes. Matching candidate columns matter, not adjacency.
No. Each deduction concerns one specific digit.
No. The board above does not promise an X-Wing in every puzzle.