Windmills are simple mechanisms that have been in use since ancient times. There are two basic designs for grain windmills, with both designs being used to grind grain into flour by capturing the power of the wind.
Windmills first appeared in ancient Persia between 500 and 900 A.D. Their original functions were to pump ground water to the surface and to grind grain. These original windmills featured vertically oriented sails used to turn a millstone. The more common horizontally oriented windmills did not appear until around 1270 A.D. in Europe
Vertical Axis System
The first Persian windmills utilised a vertical axis system, which is not the most efficient design, since only half of the windmill surface area can be used to harness wind power. This is due to the fact that the rotor mechanism needs to be adequately shielded from the incoming wind to keep the sails rotating
Horizontal Axis System
Horizontal axis windmills first appeared in Europe several hundred years after the original Persian vertical axis windmills. This type of windmill is also known as a post mill because the blades of the sail are fixed to a central post. According to Windmillworld.com, the central post allows the mill to rotate in order to face the varying wind direction. Since the entire surface area of the sails can be utilised at one time it is a much more efficient design than vertical axis windmills. It is unclear if horizontal mills were an improvement upon the Persian design brought back from the Crusades, or if they were developed independently from the similar water mill design.
If a mill used a vertical axis system or a horizontal axis system, the functional unit of the mill was still the millstone since one of the primary functions of the mill was to grind grain. The millstone or grindstone was attached directly to the vertical shaft in a vertical axis system, while horizontal axis systems used wooden gears to utilise the motion of the horizontal mill. Millstones were selected from specific quarries called millstone quarries. Specific stones needed to be used in order to maintain the proper speed and hardness of the millstone or else the grain would not be ground properly.
Telosnet.com, a website dedicated to the early history of wind power, states that sails and sailing technology contributed to the development of windmills in that the first windmills worked on the same principles as sails and sailing for harnessing wind power. The first windmill sails were made from reeds or wood.