A double check is a discovered check where the moving piece ALSO gives check, two checks arriving in the same instant. It has a unique property: blocking can only stop one check, and capturing can only remove one checker, so against a double check the king itself must move. No other reply is legal.
That makes double check the most forcing move in chess, and the engine behind many famous mating attacks. If every square the king could run to is covered or blocked, a double check is instantly checkmate, even when both checking pieces could otherwise be captured.
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White's knight on d7 blocks the bishop's path to e8. Black's king is packed in by its own pieces.
Find the double check. It happens to be mate.
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