Elevators are dispatched one-by-one when they reach a pre-determined passenger load, or when they have had their doors opened for a certain period of time. For buildings of much over seven stories, traction elevators must be employed instead. The weight of the car is balanced with a counterweight. Later Otis designed a special elevator for the school. In this mode, an elevator will stop automatically at every floor, allowing people to step on and off without having to press any buttons. The engine only needs to overcome inertia and compensate for the varying weight of the passengers in the cars. Japanese or lift e.