Passed Pawns Weak in Double Rook Endings
A single passed pawn is often not a big advantage in a double rook ending. In fact, an outside passed pawn is quite weak, is often simply lost, and at best will be blockaded with no possibility of progress.
This is an exception to the general rule that outside passed pawns are strong in endgames. The problem in double rook endings is that two rooks can easily blockade a single passed pawn, either both defending rooks in front of it, both behind it, or both defending rooks can also blockade it horizontally. If both defending rooks are attacking the pawn (from the front or behind), then both of your attacking rooks have to sit there defending the pawn, making them both passive, and the pawn cannot move even one square forward once blockaded by an enemy rook.
In a single rook endgame, you would then bring your King over to help out. But in a double rook endgame, the King is just too exposed. It will get constantly checked, and in some cases, is at risk of some tactical tricks.
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