Learn / Wings
Sudoku technique · Wings · first needed in Tricky puzzles
XY-Wing / Y-Wing
A two-candidate pivot XY sees wings XZ and YZ. One wing must be Z, whichever value the pivot takes.
How to spot it
Find a bivalue pivot and two suitable bivalue neighbors.
The common mistake
An elimination must see both wings. It need not see the pivot.
Learn this first
Bivalue cells.
A worked example
This position comes from SudokuByte’s lesson on the XY-Wing. Gold cells hold the evidence; green cells are the pattern’s ends.
25
28
4568
7
1
89
346
2569
23459
1257
3
158
6
4
89
17
1259
2579
17
9
146
3
2
5
1467
8
47
2359
28
589
49
38
1
3468
7
23489
4
6
7
5
38
2
138
19
389
239
1
89
49
7
6
5
29
23489
19
7
19
8
5
3
2
4
6
6
5
2
1
9
4
78
3
78
8
4
3
2
6
7
9
5
1
25
28
4568
7
1
89
346
2569
23459
1257
3
158
6
4
89
17
1259
2579
17
9
146
3
2
5
1467
8
47
2359
28
589
49
38
1
3468
7
23489
4
6
7
5
38
2
138
19
389
239
1
89
49
7
6
5
29
23489
19
7
19
8
5
3
2
4
6
6
5
2
1
9
4
78
3
78
8
4
3
2
6
7
9
5
1
Where to look
Find a bivalue pivot and two suitable bivalue neighbors.A two-candidate pivot XY sees wings XZ and YZ. One wing must be Z, whichever value the pivot takes.
Read the evidence in this position
The pivot r6c3 contains 8, 9. Its wings are r4c2 with 2, 8 and r6c8 with 2, 9. Each wing sees the pivot.Why the deduction must follow
Try each pivot candidate. If the pivot is 8, r4c2 cannot be 8 and must be 2. If the pivot is 9, r6c8 cannot be 9 and must be 2. Therefore remove 2 only from cells seeing both wings.Apply the result
Erase 2 from r4c9, r6c1. The pattern may remove candidates without immediately solving a cell.
Further reading: hodoku.sourceforge.net