Tier 4 · first asked for by Diabolical
W-Wing
Two matching pairs joined by a strong link force one of them to take the other digit.
Also called W-Wing.
Find two cells, not seeing each other, that hold the same two candidates {X,Y}. Then look for a row, column or box where X has exactly two possible cells — a strong link — with one of those cells seeing the first pair cell and the other seeing the second.
One end of that strong link is X. Whichever end it is, it blocks X from the pair cell it sees, forcing that cell to be Y. So one of the two pair cells is Y, and Y can be removed from everything they both see.
A worked example
A real position, with the pencil marks a solver would have at that point.
the pattern · what it rules out · 5 a candidate that goes
What it proves. W-Wing: R7C7 and R8C2 both hold (15), bridged by the 5-pair in row 1, so one of them is 1 and 1 leaves R8C7, R8C8.
Builds on Naked Pair.