Double-deck elevators are designed with two elevator cars that are attached, one on top of the other. This allows passengers on two consecutive floors to be able to use the elevator simultaneously, significantly increasing the passenger capacity of an elevator shaft. Such a scheme can prove efficient in buildings where the volume of traffic would normally have a single elevator parking lift stopping at every floor.

An example, a passenger may board one elevator, which serves only odd-numbered floors on the third floor while another passenger may board another elevator, which serves even-numbered floors on the fourth floor. The elevator serving even floors is actually on top of the elevator serving odd floors in the same lift shaft. When a passenger disembarks from the even-floor-serving elevator at level 30, for instance, the passengers in the odd-floor-serving elevator beneath it are kept waiting until the elevator doors above close.

Architecturally, this is important, as double-deck elevators occupy less building core space than traditional single-deck elevators do for the same level of traffic. In skyscrapers, this allows for much more efficient use of space, as the floor area required by elevators tends to be quite significant. The other main technique is shared-shaft elevators, where multiple parking solution elevators use different sections of the same shaft to serve different floors, with skylobbies separating the sections.

Not all double-deck elevators are used to transport passengers simultaneously in both decks. Sometimes one or more elevators in a building have a double-deck car, where the second deck is used for transportation of goods, typically outside of peak traffic periods. This technique has the advantages of preventing damage to interior fixtures due to impact from trolleys, and does not require a dedicated shaft solely devoted to a goods-only scissor car elevator car. During peak periods, the car is switched back to passenger mode, where it can expedite passenger movement into or out of the building.