Fantail windmill on a smock mill in open country. A smock mill is a type of windmill with the sails borne on a cap on a tower. The fantail, a smaller windmill behind the main sails, is designed to ensure the main blades are facing into the wind. The wind blows the smaller windmill, which turns gears which turn the cap on which the main sails are attached. When the wind is parallel to the fantail it no longer provides this power, leaving the main sails perpendicular to the wind. Filmed in Denmark.