A bishop covers the king's escape square in the corner while a rook or queen comes down the open h-file to give mate.
Why it matters
A bishop covering g8 from far away turns an open h-file into a mating attack. The rook or queen comes down the file, the king cannot step aside because of the bishop, and its own pawn on g7 blocks the last square.
It shows why the a2–g8 diagonal is worth so much in attacks against a castled king: the bishop never moves, and it is the reason the mate exists.
See it on a board
The bishop on b3 never moves and never captures anything. It covers g8, which is the only reason the rook's check on the open h-file is mate.
Checked: this line ends in checkmate.
How to play it
- Get a bishop onto the a2–g8 diagonal with the squares in front of it clear.
- Open the h-file — usually by trading or pushing the h-pawn.
- Bring the rook or queen down the file. The king has no way out if its pawn is still on g7.
At each level
The same idea, said the way it is taught at each age.
Rook · ages 6–9
Before you look for the mate, look at the long diagonal. A bishop aiming at g8 from b3 or c4 is often the piece that makes the attack work.
Queen · ages 8–11
Defensively, this is the reason to be careful about opening the h-file when the opponent has a bishop on the a2–g8 diagonal, even when the file looks harmless.
What goes wrong
- Opening the h-file before the bishop's diagonal is clear, so the king escapes to g8.
- Trading the bishop off because it is 'doing nothing' — it is the whole mate.
Questions people ask
- Does the black pawn have to be on g7?
- Something has to take g7 away from the king. Usually it is the pawn; occasionally it is a piece, or your own attacker covering the square.