Sudoku Tutorial
XYZ wing versus XY wing
XYZ wing XY wing (rectangular)
XYZ1.jpg Compare:   Assume candidate Z gets eliminated from target cell(s)
XY wing Base contains 2 candidates
Wings (pincers) are YZ & XZ The four involved cells  (including target) can reside in four separate boxes  (rectangular) Target cell must see both wings but not necessarily see the base 
XYZ wing Base contains 3 candidates
Wings (pincers) are YZ & XZ Base must share a box with  a wing (never rectangular) Target cell(s) must see both wings and the base

XY wing (non rectangular)

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