Windmill

A windmill is a calculated sequence in which two coordinated pieces force the enemy king to move among very few squares. One piece gives check, directly attacking the king. When it moves away, it opens the attack of a piece behind it; this is called a discovered check. The front piece can then return with another direct check and repeat the cycle.

The classic form uses a rook and bishop. The bishop maintains a line toward the king while the rook moves away to uncover check, captures a piece or changes square, and then returns with check. Because the king must respond repeatedly, the rook may capture several pieces without giving the opponent a free tempo, meaning one move, to save them. The back-and-forth motion explains the names windmill and seesaw.

For the mechanism to repeat, the discovered-check line must remain available, the king must have few legal squares, and the front piece must be able to return with check. An intermediate defense, the capture of an attacker, or a newly available escape square breaks the cycle, so every repetition must be calculated separately.

Windmills are rare, but they may win several pieces, lead to checkmate, an attack on the king with no legal answer, or save a game through a continuous series of checks. A preliminary sacrifice, an intentional offer of a piece, is sometimes needed to draw the king into a position where its escape squares are controlled.