Greco's mate is a checkmate delivered by a rook or queen on the open h-file, with a bishop covering g8 and the defender's own g-pawn shutting the king in.
The pattern is named after Gioachino Greco, the 17th century Italian who catalogued more mating attacks than anyone of his era. His signature finish: a bishop on the a2 to g8 diagonal pins the king in place while a heavy piece drops onto the open h-file.
The key ingredients are the defender's own g-pawn, which denies the king g7, and the bishop, which covers g8. Once the h-file is open, a rook or queen check on that file is instantly mate. The h-file is usually pried open with a sacrifice: a knight or bishop takes on h7 or g6, or the attacker simply marches the h-pawn up the board and trades it.
For defenders the lesson is about the g-pawn: after moves like g6, your king needs the g7 square guarded and the h-file kept closed. An attacker's bishop staring at g8 plus a half-open h-file is the exact Greco storm warning.
Usually by force: a piece sacrifice on h7 or g6 drags a pawn off the file, or the attacker pushes the h-pawn until it trades itself. Once the file is open, the mate threat appears immediately.
Keep a piece able to block the h-file or challenge the diagonal bishop. The mate only works while g8 is covered and g7 is blocked, so trading the light-squared bishop off or freeing g7 for the king defuses it.
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