The Square Rule
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pawn on its way to Promotion and if the opponent's king can get to it before Promoting
Are you asking what it is?
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The pawn is four squares away from promoting, so you can make an imaginary 4x4 square with the pawn in the bottom right corner. If the king is in the square, it's in time to stop the pawn. If the king is outside the square, the pawn can promote easily.
