I have been asked this question in unit test.
The origin of the question is Parts of Window in portion Doors, Windows and Ventilators of Construction & Building Materials
Easy explanation: The revolving shutters or leaves which are 4 in numbers are radially attached to the pivot. The shutters may be fully glazed, fully panelled or partially panelled and partially glazed. The central pivot is provided with the ball bearing at the bottom and Bush bearing at the top so that its rotation is smooth and without any jerks, friction and noise.