Pins occur when one piece “pins” an enemy piece against the King, Queen, or occasionally another piece. Pins can occur along diagonals (by Bishops and Queens) or along ranks or files (by Rooks or Queens). You cannot do a pin with a pawn, Knight, or King.
Pins work by trapping one enemy piece against another. The pinned piece cannot escape, because if it moves away, the piece behind it is under attack. So a pinned piece can be captured immediately and won. Or sometimes, if the pinned piece is protected by another piece, you can “double up” on a pinned piece by attacking it with a second of your pieces, before winning it.
Diagram: Black Queen Pinned by White Rook
Diagram: White Queen pinned by Black Bishop is lost!
Related Chess Tactics
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