An industrial monument in which a set of machinery dedicated to a particular purpose, usually grinding corn, is powered by the motion of a wheel to which a series of sails are attached so that it can be turned by the force of the wind, the machinery itself being housed in some kind of structure around or below the aerially mounted sails. In northern Europe windmills have been recorded from the 12th century ad onwards, and many remain in a form not unlike those common in the Middle Ages. By the 17th century windmills were a common sight in town and countryside. The earliest types were postmills, where the whole structure revolves around a central wooden post rising from a ground frame comprising two intersecting beams known as cross‐trees. These cross‐trees were sometimes set in a round mound of earth and stone or rested on brick or stone piers. Above the ground frame was a superstructure containing the machinery, which pivoted on the central post so that the whole thing could be turned to move the sails into an optimum position to take full advantage of the wind. From the 14th century towermills began to appear (also called smockmills), in which the greater part of the structure was stationary with only the cap at the top, carrying the sails, able to be rotated into the wind. Towermills were therefore far more substantial, usually of timber on a brick or stone base, but occasionally having the whole tower of brick or stone. Archaeologically, early windmills of both main types are represented by low earth mounds or circular foundations.