XYZ-Wing
tough
This is a variation of an XY-wing. In the partial puzzle below, consider the cells that have only the candidates shown. Any cells that share a container with all three cells XYZ, XY and YZ, can have Z eliminated from their candidates.
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It can be easily seen that which ever value is in XYZ, the cells marked with the asterisk cannot be Z.
This allows Z to be eliminated from the candidates for the marked cells.if XYZ = X, then XZ = Z, so * cannot be Z
if XYZ = Y, then YZ = Z, so * cannot be Z
if XYZ = Z, then * cannot be Z
In the Sudoku puzzle below, the XYZ-wing in the green cells allows 1 to be eliminated from the blue-marked cell.