An automatic transmission (commonly abbreviated as "AT") is an gearbox that can change gear ratios automatically as the vehicle moves, thus freeing the driver from having to shift gears manually (similar but larger devices are also used for railroad locomotives). Most automatic transmissions have a set selection of possible gear ranges, often with a parking pawl feature that will lock the output shaft of the transmission. The other type that has become more comon is the TorqueConverter, or Hydrostatic transmissions in machines such as Combines, Excavators, Forklifts, and other plant.