Ford also had a similar electro-hydraulic system on higher-end convertibles. Mercury and Ford Sportsman convertibles (with wood trim) 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.