W-Wing
Two cells with the same pair, joined by a strong link — the digit not in the link falls anywhere seeing both.
How it works
- 1Find two cells, not seeing each other, whose candidates are the same pair {a,b}.
- 2Find a house where a has exactly two homes left (a strong link), one seeing the first cell and one seeing the second.
- 3One end of that link must be a, which forces the cell it sees to be b — so the two cells cannot both avoid b.
- 4Any cell seeing both of them therefore cannot be b.