Windmill is a solitaire card game played with two decks of playing cards. It is so called because the initial layout resembles a windmill's sails.

First, an ace is placed at the center. Then eight cards are placed around in such a way that the layout looks like a cross. The ace forms the primary foundation and the eight cards form the reserve. The gaps at the corner of the "sails" are reserved for kings and form the secondary foundations. The suits of the kings to be placed on the secondary foundations are disregarded. The object of the game is to built the primary foundation up to king with 52 cards in it and the secondary foundations down to ace with 13 cards each in them. All foundations are built regardless of suit.

The illustration below shows how the tableau is initially laid out. A is for the primary foundation, R for the reserve and (K) for the secondary foundations, empty at first.

Game starts by moving cards from the reserve to either the primary foundation or if available, either one of the secondary foundations. Gaps at the reserve can be filled by cards from the wastepile, or if the empty, from the stock. If no more moves are possible from the reserve, the player deals the cards from the stock one at a time to search for applicable cards. The player can also move a card from a secondary foundation to the primary foundation provided that the next card to be placed at the primary foundation must come from the wastepile or the reserve.

When the primary foundation reaches a king, an ace can be placed over it. But when a secondary foundation reaches an ace, it can longer be built unless the ace is moved to the primary foundation.

The game is won when all cards are in the foundations, with 52 on the primary foundation and 13 on each of the four secondary foundations.