Diagram 5 shows the problem again.
The pawn on d4 is under attack from the
black knight. The rook on f4 is under attack
from the black queen. Both the white d pawn
and the white rook are guarded by the pawn on e3
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Diagram7 shows what would happen if white
plays exd4, capturing the knight with his
pawn. The white rook would be unguarded
In Diagram 6 black has played Nxd4, capturing the
white pawn with his knight. Now white dare not
capture the black knight with his pawn because
this would leave his rook unguarded,
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Diagram8 The black queen could now capture the
white rook Qxf4
