Automatic transmission: In the majority of cases this consists of a torque converter and a set of gears called planetary or epicyclical gear train. These are fitted to the car in place of a conventional clutch and gearbox.

The torque converter, as the name implies, converts the torque or turning effort of the engine power through the gear ratios to the drive wheels. Basically, the torque converter consists mainly of an impeller, which is driven by the engine and a turbine that drives the gearbox. Each is bowl shaped and contains a number of partitions or vanes. They are mounted face-to-face in the oil filled gearbox but there is no physical contact between them. Put simply, when you press the accelerator you increase the speed of the impeller, which forces the gearbox oil through the vanes of the turbine, making it rotate. An increase or decrease in torque has the same effect as changing to a higher or lower gear

This almost fluid connection between the engine, through the gearbox, to the drive wheels means that leaving the gear lever in 'D' when negotiating a hazard is not the same as leaving a manual gearbox in top gear.

The majority of gear selector positions are marked:
P   PARK must never be engaged whilst the vehicle is in motion as it locks the transmission and prevents the car from moving.
R   REVERSE
N   NEUTRAL
D   AUTOMATIC DRIVE USING ALL FORWARD GEARS and a configuration of 1, 2, 3 and 4 for manual selection, or a symbol such as a plus or minus sign for upward or downward manual changes. For normal driving the lever can remain in 'D' and allow the transmission to make automatic adjustments according to road speed, engine loading and accelerator position.