information:classification, structural:summarization, stylistic:style_transfer
Sails

Common sails consist of a lattice framework on which the sailcloth is spread. The miller can adjust the amount of cloth spread according to the wind and the power needed. In medieval mills, the sailcloth was wound in and out of a ladder-type arrangement of sails. Later mill sails had a lattice framework over which the sailcloth was spread, while in colder climates, the cloth was replaced by wooden slats, which were easier to handle in freezing conditions. The jib sail is commonly found in Mediterranean countries and consists of a simple triangle of cloth wound round a spar.

In all cases, the mill needs to be stopped to adjust the sails. Inventions in Great Britain in the late eighteenth and nineteenth centuries led to sails that automatically adjust to the wind speed without the need for the miller to intervene, culminating in patent sails invented by William Cubitt in 1807. In these sails, the cloth is replaced by a mechanism of connected shutters.

In France, Pierre-Théophile Berton invented a system consisting of longitudinal wooden slats connected by a mechanism that lets the miller open them while the mill is turning. In the twentieth century, increased knowledge of aerodynamics from the development of the airplane led to further improvements in efficiency by German engineer Bilau and several Dutch millwrights. The majority of windmills have four sails. Multiple-sailed mills, with five, six, or eight sails, were built in Great Britain (especially in and around the counties of Lincolnshire and Yorkshire), Germany, and less commonly elsewhere. Earlier multiple-sailed mills are found in Spain, Portugal, Greece, parts of Romania, Bulgaria, and Russia. A mill with an even number of sails has the advantage of being able to run with a damaged sail by removing both the damaged sail and the one opposite, which does not unbalance the mill.

In the Netherlands, the stationary position of the sails, i.e. when the mill is not working, has long been used to give signals. If the blades are stopped in a "+" sign (3-6-9-12 o'clock), the windmill is open for business. When the blades are stopped in an "X" configuration, the windmill is closed or not functional. A slight tilt of the sails (top blade at 1 o'clock) signals joy, such as the birth of a healthy baby. A tilt of the blades to 11-2-5-8 o'clock signals mourning, or warning. It was used to signal the local region during Nazi operations in World War II, such as searches for Jews. Across the Netherlands, windmills were placed in mourning positions in honor of the Dutch victims of the 2014 Malaysian Airlines Flight 17 shootdown.