The POST MILL is the earliest type of windmill, being introduced soon after the Norman conquest. In these mills, the whole body containing the grinding machinery and grain storage was supported on a large central vertical post. These mills were of necessity relatively small as the miller would have to turn the entire body of the mill so that the sails faced into the wind. The sails were made of a lattice construction over which canvas was spread to catch the wind similar to a sailing ship, hence the name "sails". Later, fantails were fitted so that the sails would automatically keep facing into the wind. A good example of a post mill in the south east is the Jill Mill at Clayton, just north of Brighton in Sussex.

In a SMOCK MILL, of which Cranbrook Union Mill is a fine example, the main body of the mill remains static and just the cap rotates with the sails. The lower half of the body is constructed of brick and the upper half of stout timber framing clad in lapped wooden boards, known as weatherboards. Because the mill was so much higher and the sails further from the ground to catch more wind, a gallery or walkway had to be constructed to enable the miller to gain access to the sails.