A fork is one piece attacking two (or more) enemy targets at the same time. The defender can only save one, so a well-timed fork wins material almost automatically.
Knights are the classic forking piece, since they attack in an L-shape that a queen or rook or bishop can't block by simply stepping in the way. Whenever your knight can jump to a square that hits two valuable pieces, look hard.
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A white knight on d4, with a black rook on c7 and queen on g7.
Which knight move forks the rook and the queen?
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