A Four-wheel drive (also called 4WD and 4x4) is a vehicle on which all four wheels get power from the engine to move forward. 4WDs are often used off road. A true four wheel drive vehicle uses a transfer case to direct power to one or both axles. By having a transfer case the driver can shift the vehicle into either "two-wheel-drive" or "four-wheel-drive" mode. Many transfer cases also have a low power range that multiplies the engine power when in four-wheel drive (called four-wheel low gear). A variant, or similar system, is called All-wheel drive. All-wheel drive uses a center differential to direct a percentage of the power to both axles, depending on conditions. All wheel drive is not designed specifically to go off road but is for traction in rain, snow or ice on the road.