Advanced
Y-Wing
A pivot cell and two peer wings that pinch a common digit.
Y-Wing (sometimes called XY-Wing) is the workhorse chain. The pattern needs three cells, all bivalue. The pivot has candidates {X, Y}. Two peers of the pivot — the wings — have {X, Z} and {Y, Z}.
Reason it out: whichever digit the pivot finally turns out to be (X or Y), exactly one of the wings is forced to be Z. So any cell that sees both wings cannot be Z and can drop it.
Y-Wing is where chain logic becomes natural. Most expert patterns rest on the same intuition.
When the move applies
Y-Wings appear once your bivalue cells are mapped. Look for three of them arranged as a pivot and two wings whose candidate sets form the {XY, XZ, YZ} triangle.
The procedure
- Find a bivalue cell A with candidates {X, Y}.
- Find a peer B with candidates {X, Z}.
- Find a peer C with candidates {Y, Z}.
- Eliminate Z from any cell that sees both B and C.