W-Wing
Two cells share the same candidate pair and aren't in the same row or column, but a chain elsewhere forces one of them to one of those values — eliminate the other from any cell both see.
How it works
- 1The opposite-corner cells at row 1 column 1 and row 4 column 4 share the pair {1,2}, but they do not see each other.
- 2In row 2, the two highlighted cells are the only places for 1. The left one sees the first {1,2} cell; the right one sees the second. Exactly one link end is 1.
- 3Whichever link end becomes 1 forces the {1,2} cell it sees to become 2. So at least one endpoint is 2, and the top-right target — which sees both endpoints — cannot be 2.
- Candidates this technique rules out
- Cells read together — The connecting line shows which cells combine to make the deduction.
- Conclusion — The coloured candidate, or the final arrow when it lands outside the pattern, shows the result.