A transfer case is an essential part of four-wheel-drive drive-train systems found in both all-wheel-drive and four-wheel-drive vehicles. It is connected to the car's transmission and is used to engage and disengage the car when started and stopped.
The majority of transfer cases are either gear based or chain based and are present under the vehicle as steel or aluminum housings. They receive power from the transmission system and send it to the rear and front axles. Modern transfer cases, especially the ones designed for off-road vehicles, are capable of mechanically locking the rear and front axles when required. Periodic maintenance is required to make their lifespan longer.