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 allowing large numbers of people to pass in and out. Construction Around the central shaft of the revolving door, there are usually three or four panels called "wings" or "leaves." Large diameter revolving doors can accommodate strollers and wheeled luggage racks. Some "revolving door displays" incorporate a small glass enclosure, permitting small objects such as sculpture, fashion mannequins, or plants to be displayed to pedestrians passing through. Such enclosures can either be mounted at the central pivot, or attached to the revolving door wings. The wings of revolving doors usually incorporate glass, to allow people to see and anticipate each other while passing through the door. Manual revolving doors rotate with pushbars, causing all wings to rotate. Revolving doors typically have a "speed control" (governor) to prevent people from spinning the doors too fast. Automatic revolving doors are powered above/below the central shaft, or along the perimeter. Automatic revolving doors have safety sensors, but there has been at least one fatality recorded. Skyscraper design requires a means of draft block, such as revolving doors, to prevent the chimney effect of the tall structure from sucking in air at high speed at the base and ejecting it through vents in the roof while the building is being heated, or sucking in air through the vents and ejecting it through the doors while being cooled, both effects due to convection. Modern revolving doors permit the individual doors of the assembly to be unlocked from the central shaft to permit free flowing traffic in both directions. This feature, called "breakout" or "break away", is typically used only during emergencies, or to admit oversize objects. The most effective method for this is the "bookfold" design, which allows all 3 or 4 wings to be broken away together. Normally, the revolving door is always closed so that wind and drafts cannot blow into the building, to efficiently minimize heating and air conditioning loads. In right hand traffic countries,