The Dutch are known for maximizing the usefulness of resources. Historically they used windmills to pump seawater out of lowlands, allowing them to reclaim land. Windmills were also used to power sawmills and of course, to grind grain.

Fascinatingly, the Dutch even found a way to make windmills useful when the sails weren't turning. When the brake was applied to stop the sails from rotating, the operator would leave the sails in a specific orientation, which conveyed information to everyone within sight of the mill.

If the sails were frozen in a "+" configuration, it meant "short break," meaning "open for business;" in other words, the miller was not milling grain at the moment and was free to serve customers, so now was the time to come and buy some.

Image: Von Joachim Müllerchen - User:Aeggy, CC BY 2.5

If the sails were frozen in an "x" configuration, it meant "long break," like if the mill was down for maintenance or if the miller had to travel. I.e., not open for business; if you saw the sails in this position, you were not buying grain that day.

Image: Von Joachim Müllerchen - User:Aeggy, CC BY 2.5

If the topmost sail was in the 1 o'clock position, it signified "joy" or "birth." For instance, if a family in the village was expecting a baby, and you spotted the topmost sail in this position, you'd know the baby had been born. It would also be placed in this position to celebrate a wedding.

Image: Von Joachim Müllerchen - User:Aeggy, CC BY 2.5

If the topmost sail was in the 11 o'clock position, it signified "mourning" or "death." It would be placed in this position following a funeral.

Image: Von Joachim Müllerchen - User:Aeggy, CC BY 2.5

As an example of the latter, the windmill below, photographed in 1962, is in the mourning position following the death of Queen Wilhelmina.

Image: Rijksdienst voor het Cultureel Erfgoed, CC BY-SA 4.0

As for why the 1 o'clock and 11 o'clock positions were seen to correspond with joy and mourning, respectively: In Holland, windmills rotate in a counterclockwise direction. When the topmost sail is in the 1 o'clock position, it is just about to reach its apex. In the 11 o'clock position, it has passed its apex and is descending.

The definitions above apply loosely to the whole of Holland; the positions and meanings can differ regionally, as well as in neighboring Belgium and Germany, where this passive communication system was also used. The Germans even have a word for it: Mühlensprache, meaning "mill language."