A revolving door is a way to enter or exit a building. In a revolving door, several panels are fixed on a central axis. The panel can then be rotated to enter or exit the building. Buildings are usually fitted with revolving doors to save energy: Revolving doors can be made to hinder airflow. This means that less heat is lost. The first to develop revolving doors was H. Bockhacker. In 1881, he registered a patent in Germany for a "door without airflow". Theophilus Van Kannel registered a patent for what he called "storm door", in 1888.