This is where the top of the windmill rests on the base. The sails of a windmill work best when pointed straight into the wind. Early windmills were simply built facing the prevailing wind. More advanced versions could be turned by an ox. In this fancy model, a tail rotor (visible from the outside) automatically keeps the windmill pointed into the wind by turning the gear shown here (and others like it), driving the mill top on the toothed track.