XY-Wing
A three-cell wing structure where shared candidates allow elimination from cells that see both wings.
XY-Wing uses a pivot cell with candidates {X, Y} connected to two wing cells with candidates {X, Z} and {Y, Z} respectively. Regardless of whether X or Y occupies the pivot, Z must appear in one of the wings. Therefore, Z can be eliminated from any cell that sees both wings.
Structure and Logic
The pivot has candidates {X, Y}. If X goes in the pivot, wing 1 ({X, Z}) loses X and Z is confirmed. If Y goes in the pivot, wing 2 ({Y, Z}) loses Y and Z is confirmed. Either way, Z appears in one wing. Any cell that sees both wings cannot contain Z.
Discovery Tips
Identify all bivalue cells (cells with exactly 2 candidates), then look for three that form the XY-Wing structure. The pivot must share a unit with each wing, but the two wings need not share a unit with each other. This technique appears frequently in Master difficulty and above.