Four-wheel drive, all-wheel drive, AWD, 4WD, or 4×4 ("four by four") are synonymous to All – Wheel drive with minor differences here and there. This helps in getting maximum torque at the axles. They can be selected to act as either part – time or full – time 4WD. The drive terrain of the vehicle which channels the power of the engine through the transmission unit and transfer gear box to both the front & rear differential and wheels all the time is called all – wheel drive vehicles. The Differentials and limiting slippage factors are considered while designing such vehicles. They are used for heavy trucks, road racing and in construction of some equipments. This terrain was first introduced to off – roaders as it was a boom in the market that time and now these can also be seen passenger cars. These include certain systems which are categorized on basis of their design type viz. Center differential with mechanical lock, Torsen center differential, Non-locking center differential, Multiple – clutch systems, Multi-plate clutch coupling, Multiple gearbox and Part-time. The others are different from 4WD as follows in Four-by-four (4x4) the first figure tells us the number of wheels (more precisely, axle ends), and the second tells which of them are powered whereas in an All Wheel drive (AWD) or permanent multiple – wheel drive the drive terrain system includes a differential between the front and rear drive shafts. This is generally coupled to some anti – slip technology which help the differentials to get different speeds. They have the ability to transfer torque from one wheel due to loss of traction at that wheel.