Since ancient times, people have harnessed wind power, beginning with sailboats, whose technology played an important role in the development of sail-type windmills. Windmills have existed for at least 1,300 years, and the first, built in Persia around the 7th century A.D., had vertical shafts and six to twelve sails covered in fabric or palm leaves for grinding corn and pumping water. Dutch and British windmills marked pinnacles of design, with the Dutch using them extensively to pump water and grind flour while also developing many advanced automatic control mechanisms over the centuries. Wind power continues in modern forms through wind turbines that produce electricity and small-scale wind pumps still widely used in agriculture. Wind power plants, or wind farms, are clusters of wind machines spread over large areas to generate electricity, often with dozens of machines. Horizontal-axis wind machines have blades like airplane propellers, and a typical one is as tall as a twenty-story building with three blades spanning 200 feet, while the largest have blades longer than a football field.