A revolving door typically consists of three or four doors that hang on a central shaft and rotate around a vertical axis within a cylindrical enclosure. Revolving doors are energy efficient as they, acting as an airlock, prevent drafts, thus decreasing the loss of heating or cooling for the building. Revolving doors were designed to relieve the immense pressure caused by air rushing through high-rise buildings (referred to as stack effect pressure) while at the same time, to allow large numbers of people to pass in and out.