Summary: X-Wing is a later advanced scan, after singles, pairs, triples, quads, pointing pairs, and box-line reduction. An X-Wing is one leftover digit that has only two homes in each of two rows, and those homes sit in the same two columns — a rectangle. The same lock with two columns and two rows is the other orientation. The scan does not write the digit. It strikes that digit from every other empty in the two cover lines.
Start from the cheaper scans, then lock a rectangle
Naked Singles and Hidden Singles place a digit. Naked Pairs and Hidden Pairs lock two digits to two cells and strike leftovers. After those, the cheaper house scans still come first: triples, quads, pointing pairs, and box-line reduction. An X-Wing does not place and does not lock two different digits. It locks one digit to two lines, then strikes that digit from the rest of the crossing pair.
If you can still place a single, lock a pair, or clear leftovers with a triple, a quad, a pointing pair, or a box-line, do that first. Use this page when those earlier scans stall and the same leftover keeps appearing twice in a row, then twice again in a second row, lined up. For the 9×9 rules and the first fills on a nearly-full box, see How Sudoku Works.
Count rows down from the top and columns left to right. A printed digit stays put. Blank cells are empty. In the figure, blue marks the four corners of the rectangle — what the scan reasons from — and amber marks two empties the scan strikes.
X-Wing: two rows, two aligned homes, one digit
An X-Wing is one digit that can go in only two cells of one row and only two cells of a second row, and those four cells share the same two columns. The four homes form a rectangle. That digit must occupy both columns — one corner in each column — so it cannot live in any other empty of those two columns.
You find it by staring at one digit in two rows. Pick a missing digit. In the first row, list every empty that can still take it. You need exactly two. In a second row, list them again. If you again have exactly two, and they sit in the same two columns, you have the rectangle. Strike that digit from every other empty in those two columns.
The four corners are not singles. Each corner may still list other leftovers. The lock is on the digit, not on the cells.
The same definition with the houses flipped is the other orientation: two columns, each with exactly two homes for the digit, aligned on the same two rows. Then you strike that digit from the other empties of those two rows.
Worked example
Look at 7 in row 2 and row 8. The blue corners are row 2 column 2, row 2 column 7, row 8 column 2, and row 8 column 7. Two cells the scan clears are amber: row 5 column 2 and row 4 column 7.
The four blue corners form the rectangle. Both amber cells sit inside the two columns it locks, which is why they are the ones that lose their 7.
Where 7 can go in row 2:
- Columns 1, 3, 8, and 9 are already filled.
- Columns 4, 5, and 6 sit in the top-center box, which already has a 7 at row 1 column 5, so none of those three empties can be 7.
- That leaves the two blue cells: column 2 (leftovers 2, 4, 7) and column 7 (leftovers 5, 7).
Where 7 can go in row 8:
- Columns 1, 3, 8, and 9 are already filled.
- Columns 4, 5, and 6 sit in the bottom-center box, which already has a 7 at row 9 column 4, so none of those three empties can be 7.
- That leaves the two blue cells: column 2 (leftovers 1, 7) and column 7 (leftovers 1, 3, 7, 9).
Two rows, two homes each, same two columns. That is the rectangle. One of the two blue cells in column 2 must be 7, and one of the two blue cells in column 7 must be 7. You do not yet know which corner of each column gets the 7.
Strike 7 from every other empty in column 2 and column 7:
- Row 5 column 2 (amber) — leftovers 1, 6, 7. Strike 7. Leftovers 1, 6.
- Row 4 column 7 (amber) — leftovers 1, 3, 5, 6, 7, 9. Strike 7. Leftovers 1, 3, 5, 6, 9.
- The same strike hits the other empties in those columns that still listed 7: row 3 column 2, row 5 column 7, and row 6 column 7.
The X-Wing did not write 7. It only cleared 7 off those victims. A later single or pair can use the thinner leftover lists.
This grid is a row X-Wing. Before the strike, column 2 had 7 in four cells (the two corners plus row 3 and row 5), so a column-first scan would not have fired — those cover lines still had extra homes. After the strike those extras are gone; column 2 keeps 7 only in the two blue corners, and the reverse orientation now has no victims. That is the point of “exactly two”: the base lines are the ones that already shrank to two homes. The cover lines are allowed to have extra homes — those extras are what you strike.
When it actually applies
- Exactly two homes in each of two rows (or two columns), and those homes line up on the same two columns (or rows). Four cells, a rectangle, one digit.
- Something to strike. At least one other empty in those two cover lines still lists the digit. A rectangle with no victims is true and useless — leave it.
- Not three homes. If either base line still has a third cell that can take the digit, the rectangle is not locked. Stop. The three-line version of the same idea is Swordfish; the four-line version is Jellyfish — later pages.
- Not a pair. Two cells in one house that share two leftovers is a Naked Pair (Naked Pairs and Hidden Pairs). X-Wing is one digit across two houses.
Do not guess which corner gets the digit. If the two rows’ homes do not share the same two columns, that is not a rectangle yet. Leave it and look at another digit.
Practice on a fresh 9×9
Print an Extreme 9×9, cover the solution, and finish every Naked Single and Hidden Single, every pair, and every cheaper house scan you can see — triples, quads, pointing pairs, box-line — before you start hunting an X-Wing. Extreme is the first named level that needs this scan.
Puzzle Maker Pro Sudoku prints classic 9×9 grids with a matching answer key. The generate walkthrough is Getting Started with Puzzle Maker Pro Sudoku. The free demo generates one page at a time if you want to try a board first.
FAQ
What is an X-Wing in Sudoku?
One leftover digit that has only two legal cells in each of two rows, sitting in the same two columns — or the same lock with two columns and two rows. The four cells form a rectangle. You strike that digit from every other empty in the two cover lines.
When does X-Wing actually apply?
When each of those two rows (or two columns) has the digit in exactly two aligned cells, and at least one other empty in the crossing pair still lists that digit. A third home in either base line, or homes that do not line up, is not an X-Wing.
Does an X-Wing place a digit?
No. It only removes the digit from the victims. In the example the four corners keep 7 as a leftover; the amber cells lose it. A later single or pair writes the next digit.
Do I need pencil marks?
They help. You are looking for a leftover mark that appears twice in one row and twice in another, in the same two columns. You can also find it by walking one missing digit across two rows, as in the example.
Which scan should I do first?
Singles first — Naked Singles and Hidden Singles. Then pairs, then triples, quads, pointing pairs, and box-line reduction. X-Wing is later. If those cheaper scans stall, hunt X-Wing for one digit at a time.
Where do I learn the 9×9 rules, singles, and pairs?
How Sudoku Works for rows, columns, 3×3 boxes, and the first fills on a nearly-full box. Naked Singles and Hidden Singles for the scans that place a digit. Naked Pairs and Hidden Pairs for the scans that lock two leftovers; triples, quads, pointing pairs, and box-line still sit between those and a rectangle.