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. This is as opposed to a two-wheel drive vehicle. 4WDs are often used off road. Part-time and full-time four wheel drive vehicles uses a transfer case as part of the drivetrain 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 (part-time systems). Many transfer cases also have a low power range that multiplies the engine power when in four-wheel drive (called four-wheel low gear). 