were equipped with power windows on four windows from 1946 through 1948 and Mercury and Lincoln by 1951. These systems were used by other luxury car models (Imperial and Packard) until Chrysler introduced the all-electric operation on the 1951 Imperial. The availability of power windows increased with the use of small, high-torque electric motors. General Motors also followed with full electric operation in 1954. This included four-way and then six-way seats, which were introduced in 1956. Chevrolet introduced the oddity of power front windows (only) in the 1954 model. Ford also introduced full four-door power windows in sedans in 1954. The full-sized 1955 Nash "Airflyte" models featured optional power windows. Electrically-operated vent windows were available as early as 1956 on the Continental Mark II. The 1960s Cadillac Fleetwood came standard with power front and rear vent windows, in addition to standard power side windows, for a total of eight power window controls on the driver's door panel. Modern heavy-duty highway tractors frequently have an option for power window controls; however, these are generally what is referred to as "straight air". That is, the compressed air system used for air brakes is also used for the windows. These types of trucks have long used compressed air cylinders for seat height adjustment. In a similar fashion to the electro-hydraulic system, the compressed air is merely released to lower the window and/or seat. The compressed air is then admitted to the respective cylinder to raise the window or seat. In a typical auto/light truck installation, there is an individual switch at each window and a set of switches in the driver's door or a-frame pillar, so the driver can operate all the windows. These switches took on many different appearances, from heavy chrome plate to inexpensive plastic. However, some models like Saab, Volvo, Mazda and Holden have used switches located in the center console, where they are accessible to all the occupants. In this case, the door-mounted switches can be omitted. This also removes the need to produce separate door components and wiring for left and right-hand drive variants. Operation Power windows are usually inoperable when the car is not running. This is primarily a security feature. It would be a simple thing to allow electric power windows to be operable when the ignition is turned off, however it would also make the car much easier to steal. Some systems offer the compromise