The windmill, one turn at a time

A rook swings off a diagonal, the bishop behind it gives check, the rook swings back with check, and the whole thing goes round again. The defender watches their position get eaten and cannot do a thing.

Also called a see-saw
Runs on discovered check
Famous example Torre against Lasker, Moscow 1925

The windmill, sometimes called a see-saw, is a machine rather than a single tactic. It needs a bishop aiming at the enemy king through a square, and a rook standing on that square. When the rook moves away, the bishop's line opens and the king is in discovered check. The king has to answer the check, which means the rook is free to take whatever it landed on. Then the rook comes back, giving check itself, and the king is pushed back to where it started.

Nothing about that is optional for the defender, and that is the point. A king in check must deal with the check. It cannot capture the rook, block the diagonal, or save the pieces the rook is eating, so the machine runs until the attacker decides to stop it.

Nineteen plies, and Black never has a second option

White to move. The bishop on f6 is aimed at h8 and the rook is standing in the way on g7, which is exactly the arrangement the windmill needs. The rook is safe there because the bishop defends it, so the king cannot take it.

1.Rxf7+ Kg8 2.Rg7+ Kh8 3.Rxc7+ Kg8 4.Rg7+ Kh8 5.Rxb7+ Kg8 6.Rg7+ Kh8 7.Rxa7+ Kg8 8.Rg7+ Kh8 9.Rg5+ Kh7 10.Rxh5+

Four pawns and the queen, and at no point in that sequence did Black have a second legal move. Every single reply was forced, which is why the rook could take on f7, c7, b7 and a7 without ever being captured. The last turn of the machine, 9.Rg5, is the same discovered check pointed at the queen instead of at a pawn.

Play through it and watch the king: h8, g8, h8, g8, back and forth eight times. It never gets a choice because h7 is covered by the rook whenever the rook is on the seventh rank, g7 is covered by the bishop, and f8 is occupied by Black's own knight. The whole machine is built out of those three facts.

White could have taken the queen on move one and finished. Running the windmill first collected four extra pawns, and the queen was still there at the end, because Black never had a move in which to save it.

What a windmill needs

A bishop with a clear line to the enemy king, blocked only by your own rook. That pairing is a battery.
A rook on the square in between, defended by the bishop, so the king cannot capture it.
An enemy king with exactly one flight square, so every check is answered by shuffling between two squares.
Material lying on the rank the rook swings along, usually the seventh rank.
No enemy piece able to block the diagonal or capture the bishop. Check this one carefully. It is the usual reason a windmill turns out to be an ordinary discovered check.

Black to move. One turn of the same machine, from a real game. The check does not come from the piece that moves, which is what makes it impossible to answer and the moving piece impossible to catch.

Show the answer

43.Rb6 43...e4+ 44.Rxd6+ 44...Kxd6

The line ends with the pattern paying off in material. The position is from a real game on Lichess, rated 898 in their puzzle pool, and it is one of the positions in the Discovered Attacks skill in the trainer.

How they turn up

Almost always after a sacrifice. Somebody gives up material to plant a bishop on a long diagonal aimed at a castled king, then a rook arrives on the seventh rank next to it, and the position is suddenly worth more than the piece it cost. The most famous example is Carlos Torre against Emanuel Lasker, Moscow 1925, where the machine collected four pawns and then the queen, in that order.

Two things it is worth not confusing it with. A double check is a single move where two pieces give check at once, which forces a king move and nothing else; a windmill uses ordinary discovered checks, one at a time, over and over. And a windmill is not a mating attack. It usually ends with the attacker several pawns and a piece up rather than with mate, which is more than enough.

Questions people ask

What is a windmill in chess?

A repeating discovered check. A rook sits on a diagonal in front of its own bishop, next to the enemy king; when it moves away the bishop gives check, so the rook can capture with impunity, and when it returns it gives check itself and drives the king back. The cycle repeats, collecting material every turn, and the defender never gets a free move.

How do you stop a windmill?

You stop it before it starts, because once the checks begin there is no defence. That means not allowing an enemy bishop to settle on the diagonal pointing at your king, keeping the seventh rank guarded, and making sure your king has more than one flight square. A piece that can interpose on the diagonal breaks the machine, so keeping one is worth real material.

Is a windmill the same as a see-saw?

Yes, they are two names for the same pattern, and see-saw is arguably the better description: the king rocks between two squares while the rook swings out and back, out and back. Windmill is the older and more common name in English chess writing, and it captures the other half of the picture, which is that the thing goes round and round harvesting everything it passes.

Windmill sits under Words you hear at the board in the reference. Nearby: