In high-rise buildings of the late 19th century, regular exterior doors are hard to open because there is a slight vacuum caused by air flowing upwards through stairwells, elevator shafts and chimneys. So a Philadelphia inventor named Theophilus Van Kannel (1841-1919) came up with a new type of door that was easy to open and also saved heat in the winter: the revolving door.

Besides preventing the rapid influx of cold air into warm buildings, the invention kept out street noises and fumes. Van Kannel patented his invention (patent no. 387,571) on August 7, 1888.

The following year, he received the Franklin Institute's John Scott Medal for the usefulness of his invention to society. He then began manufacturing revolving doors in Philadelphia, installing the first one in a local building.

Revolving doors were soon found in all manner of buildings all over the world, including the Waldorf Astoria in New York and the Hotel Regina in Paris. The Van Kannel Company motto was "Always Open, Always Closed."

The International Steel Company of Evansville, Indiana, eventually bought Van Kannel's firm, but kept the design. Thousands are still in use today and the company survives as the International Revolving Door Company.