From Sudopedia, the free Sudoku reference guide
W-Wing
From Sudopedia
A W-Wing is formed by a chain of four cells:
- A: A cell with two candidates W and X.
- B: A cell with X as a candidate.
- C: A cell with X as a candidate.
- D: A cell with two candidates W and X.
It is required that A sees B, C sees D, and B and C has a strong link on the digit X. Then W can be eliminated from any cell that is seen by both A and D.
Suppose T is a cell that is seen by both A and D, and T contains W as a candidate. Then, the W-Wing can be written as follows in Nice Loop notation:
[T]-W-[A]-X-[B]=X=[C]-X-[D]-W-[T] => T<>W
[edit]
Example
In this grid, note the r5c5 and r6c3 cells with the candidates 2 and 7. Also, the green 7 candidates form a chain, with r5c8 and r6c8 forming a strong link (because r5c8 and r6c8 are the only cells with candidate 7 in this box). Hence, we can eliminate the digit 2 from all cells that are seen by both r5c5 and r6c3, i. e., those marked in red.
[edit]
