W-Wing is a chaining technique in nature. It consists of two bivalue cells with the same candidates, connected by one of the candidates with a strong link. The other candidate can be removed from all other cells that see both the bivalue cells.

Let's first look at an example.

[E9] and [I7] are two cells with bivalue candidates 4 and 7. These two cells are connected by [E8] and [I8] on candidate 7. We have: [E9]=4 -> [E9]<>7 => [E8]=7 -> [I8]<>7 => [I9]=7 Or [E9]=7 -> [E8]<>7=>[I8]=7 ->[I9]<>7 => [I9]=4. So either [E9]=4 or [I7]=4. In either case, [F7], [G9] and [H9], which see both [E9] and [I7] can not have 4 placed. Thus, 4 can be eliminated from those cells.

Below is another example.

The two bivalue cells of the same candidates 2 and 6 at [A4] and [D6] are connected by [H4] and [H6] on candidate 2. If [A4]=2, then [D6]=6; or if [A4]=6, then [D6]=2. Therefore, all cells that sees both [A4] and [D6] cannot have 6 placed. Of course, all cells that sees both [A4] and [D6] cannot have 2 placed either. But no such cells can be found since [A4] and [D6] are strong linked on candidate 2 by [H4] and [H6].

More examples can be found below:

See also: ««  Direct Elimination Techniques  »» ««  Candidates Elimination Techniques  »»