round the center shaft of the revolving door, there are usually three or four doors called "wings" or "leaves". Large diameter revolving doors can accommodate strollers and luggage racks. The tallest revolving door currently is about 16 feet (4.9 m) high with 4 wings. The glass doors allow people to see and anticipate each other while walking through. 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.