Windmill
A windmill is a repeating cycle of checks — usually a rook and a bishop working together — that harvests material one piece at a time while the enemy king is completely helpless to stop it. Each swing of the rook captures something and delivers check; each retreat delivers check again from a different piece. The king shuffles back and forth between the same two squares while the board is stripped bare around it.
The two checks that make the cycle work
A windmill needs a bishop and a rook set up so that the bishop's diagonal runs straight at the king but is blocked by the rook itself. That gives the rook two completely different ways to check the same king on alternating moves: when the rook moves OFF the diagonal to grab a pawn or piece, it uncovers a discovered check from the bishop; when the rook moves BACK onto the diagonal, it blocks its own bishop again but delivers a direct check of its own, because it's now sitting right next to (or in line with) the king. Two different checking mechanisms, taking turns, is what keeps the king from ever getting a free move to do anything else.
Starting position. A rook and bishop battery repeatedly checking a boxed-in king (illustrative position, not from a recorded game).
The rook alternates between capturing on its way out (discovered check from the bishop) and returning to block its own bishop (direct check from the rook).
Why the king can never just take the rook
The obvious defensive idea — just capture the rook when it's close enough — never works, and the reason is worth understanding rather than memorizing: every square the rook parks on next to the king is defended by the bishop along that same diagonal it's been using the whole time. Capturing would mean moving the king into a square the bishop still attacks, which is illegal. The rook is only ever exposed on squares the king genuinely cannot reach, and it never lingers on a square it could reach in the first place.
How the material actually adds up
The capturing half of each cycle is where the material comes from, and it compounds quickly precisely because it's repeatable: a single capture-and-check might only win a pawn, but a windmill isn't one tactic, it's the same tactic played several times in a row against a king that has no way to interrupt it. By the time the king finally has a legal move that isn't forced, several pieces or pawns can be gone, often far more material than the initial setup cost to create.
How the cycle finally stops
A windmill isn't infinite — it ends the moment there's nothing left on the capturing side of the board for the rook to take, or the moment the king finds a third square that neither the rook nor the bishop attacks, breaking the two-square shuffle. Recognizing this in advance matters for the attacking side too: before starting the sequence, it's worth counting exactly how much material is actually available to be swept up along the rook's path, since a windmill that runs out of targets after only one swing might not have been worth the piece invested to set it up.
The alternating checks in a windmill are built entirely from discovered check on one half of the cycle — this page assumes that idea and builds on it, rather than re-explaining what makes a discovered check forcing in the first place.
Frequently Asked Questions
Does a windmill always involve a rook and a bishop specifically?
That's the most common pairing because a bishop's long diagonal combines naturally with a rook that can step on and off it, but the same mechanism works with a queen in either role, or occasionally a rook and rook on a shared file and rank. What matters is having two pieces where one blocks the other's checking line and can move off it repeatedly.
Why doesn't the defending side just block the discovered check?
In a genuine windmill position, the king is deliberately boxed in by its own pieces or pawns with no piece available to interpose, which is exactly what makes the two-square shuffle forced rather than optional. If a block were available, the defender would use it, and the position wouldn't be a true windmill in the first place.
Is a windmill worth setting up if it only wins a pawn or two?
Not usually on its own — the setup move that creates a windmill is often a real sacrifice, so the cycle needs to collect enough material or lead into a mating attack to be worth it. Counting the available targets on the rook's path before committing to the sacrifice is the right way to judge this, rather than assuming the pattern is automatically winning just because it looks dramatic.