[1] The word "quattro" is a registered trademark of Audi AG, a subsidiary of the German automotive enterprise, Volkswagen Group.

Due to the nomenclature rights derived from the trademark, the word quattro is now always spelled with a lower case "q" by the manufacturer, in honour of its former namesake.

[2] In that original quattro system, later found in road-going passenger cars, the engine and transmission are situated in a longitudinal position.

This allowed engine torque to be automatically directed to individual axles as driving conditions and grip warranted.

The fully automated mechanical nature of the Torsen centre differential helps prevent wheel slippage from occurring, by diverting torque instantly, without any discernible notice to the vehicle occupants,[4] to the axle which has more grip.

In comparison, viscous coupling and electronically controlled centre differentials that are used in other four-wheel drive systems are reactive, since they only redirect torque after wheel slippage has occurred.

The Torsen-based quattro system also offers an advantage in the opposite function of distributing torque to the road wheels, namely engine braking.

The Torsen-based quattro-equipped vehicle is able to execute a more stable high-speed turn under deceleration, with less risk of losing control due to loss of grip in the front or rear axles.

As with the T1 Torsen, torque will be distributed dynamically depending on tractive conditions, but with an actual (rather than nominal) static bias.

Whilst this is superficially the same as a normal open differential, adapted for a centre application, it has some key differences: If one axle loses grip, different rotational speeds arise inside the differential which result in an increase in axial forces that force the clutch plates to close.

How the system performs: When all differentials are unlocked, the car will not be able to move if one wheel (front or rear) loses traction (is on ice or raised in the air).

How does the system perform: In on-road conditions the car will not be able to move if one front and both rear wheels lose traction altogether.

Torsen type 1 centre differential, 50:50 'default' split, automatically apportioning up to 75% of torque transfer to either front or rear axle.

[5] The torque vectoring rear axle differential is designed and manufactured by Magna Powertrain,[7] and is being offered on Audi A4, A5, A6 and their derivatives (including RS models).

The Sport Differential selectively distributes torque to the rear axle wheels thereby generating a yaw moment, which improves handling and also stabilizes the vehicle when it oversteers or understeers, thereby increasing safety.

[8] Since Volkswagen Group's first mainstream transverse engined vehicle in 1974, four-wheel drive (4WD) has also been considered for their A-platform family of cars.

Attached to the transaxle is a Power Transfer Unit (PTU), which is connected to a rear axle through a propeller shaft.

Starting with the Mk4 generation A4-platform, the viscous coupling has been dropped in favour of a Haldex Traction electro-hydraulic limited-slip "coupler" (LSC) or clutch.

If one side of the driven axle loses grip, then the Electronic Differential Lock (EDL) component of the ESP controls this.

The aforementioned viscous coupling 4WD system was found in the Mk2 generation of transverse-engined A2-platform vehicles, including the Volkswagen Golf Mk2 and Jetta.

The Vanagon system was RWD-biased, the engine and transaxle were in the rear, whereas the viscous coupling was found in the front axle near the final drive.

The coupling locks when slipping occurs and up to near 50% of torque is automatically transferred to rear axle (front in Vanagon).

For four-wheel drive to work when reversing, a vacuum-actuated "throttle control element" is installed on the differential case.

Curiously, the Bugatti Veyron also utilizes Haldex, though with separate transmission, PTU and front and rear axles.

Haldex Traction LSC multi-plate clutch with ECU electronic control, acting as a pseudo center differential.

A Haldex Traction LSC unit may divert up to a maximum 100% of the torque to the rear axle as conditions warrant.

On all transverse-engined cars with the Haldex Traction LSC four-wheel drive system, the EDL only controls the front wheels, and not the rear.

Again, due to limitations of Electronic Differential Lock (see quattro IV description above), in off-road conditions it is enough for one front and one rear wheel to lose traction and the car will not move.

When performing tight low-speed turns (e.g. parking) the clutch is disengaged by Electronic Control Unit to avoid "wind-up" in transmission.

As part of Audi's celebration of quattro All-Wheel Drive technology, a TV commercial titled 'Ahab' was produced inspired by the American novel, Moby Dick.