The post mill is the earliest type of European windmill. Its defining feature is that the whole body of the mill that houses the machinery is mounted on a single vertical post, around which it can be turned to bring the sails into the wind. All post mills have an arm projecting from them on the side opposite the sails and reaching down to near ground level. With some, as at Saxtead Green, the arm carries a fantail to turn the mill automatically. With the others the arm serves to rotate the mill into the wind by hand.