Sudoku techniques · Expert
X-Wing in Sudoku
An X-Wing is the first pattern that reaches across the whole grid. One number appears as a candidate in exactly two cells of one row, in exactly two cells of another row, and all four cells line up in the same two columns. Whichever way the number falls, it takes one cell from each column, so those two columns lose it everywhere else.
Also known as: Basic Fish, 2-Fish.
Why it works
When a number is locked to the same two columns in two rows (or to the same two rows in two columns), that candidate is removed everywhere else in those two crossing lines.
How to spot it
Pick one number and map where it can go across the whole grid, one row at a time.
- You want two rows in which the number has exactly two spots each. Not three, exactly two.
- If those two pairs sit in matching columns, you have an X-Wing, and both columns clear.
- Then do the same scan by column to find the transposed version. Most solvers only ever check one direction.
Builds on
Worked examples
These are the app's own lesson boards. Step through them the same way a hint does.
Row-Based
A candidate appears in exactly two columns across two rows.
- 1
In each base row, 4 appears as a candidate in only two cells. These cells line up in the same two column positions, forming an X-Wing!
- 2
The X-Wing means 4 must occupy one of the two diagonal pairs of these four corners. So in the two columns that hold them, we can remove the candidate 4 from every cell outside the two base rows.
Column-Based
A candidate appears in exactly two rows across two columns.
- 1
In each base column, 4 appears as a candidate in only two cells. These cells line up in the same two row positions, forming an X-Wing!
- 2
The X-Wing means 4 must occupy one of the two diagonal pairs of these four corners. So in the two rows that hold them, we can remove the candidate 4 from every cell outside the two base columns.