Three sub-terms are defined under this heading: Full time 4WD, Part Time 4WD, and Permanent 4WD. FULL TIME 4WD: The most commonly used 4 WD system on the market. Full time 4WD offers both a two-wheel drive mode for summertime driving or dry-road conditions, and an automatic 4WD mode for changing road conditions. There is also 4WD high and 4WD low modes for when the going really gets tough. The automatic 4WD mode makes this system convenient for many drivers. In addition to the transmission and transfer gearbox, a center differential is used to couple the front and rear wheels. The differential allows the front and rear wheels to turn at different speeds as needed, letting the full time 4WD work automatically. Very simply, when engaged in automatic 4WD on a dry straight road, the system operates in two-wheel drive. When the wheels start to spin due to slippery road conditions, the system reacts by progressively locking the front and rear wheels together to optimize traction. This system is limited in that it requires the driver to determine when to engage it. PART TIME 4WD: This is the most basic of all 4WD systems. It gives the driver the ability to choose when to drive in two-wheel drive or 4WD. This is all good and well, until you realize that you really can't engage the 4 WD on pavement unless it is very, very slippery. That is because, when you engage 4WD with this system, you lock the front and rear wheels together through the transmission and transfer gearbox. This is great for straight-ahead traction on very slippery surfaces. However, on dry pavement it makes for odd driving, cornering, and handling characteristics. Also, you can harm the drive train components by driving in 4WD for extended periods while on dry pavement. So why choose this type of system? Three reasons: 1. It is less costly to build and therefore to purchase 2. It is often very durable under heavy stress 3. When you don't need 4WD, you can disengage it. PERMANENT 4WD: This system is similar to full-time 4WD, but it has no two-wheel drive mode. The vehicle is always in 4WD, so you don't have to determine whether conditions are right to engage it. It still has transmission, transfer gearbox, and center differential coupling the front and rear wheels. The only difference is that torque (power) is constantly being applied to all the wheels, giving maximum traction in all weather and road conditions. Current systems have high and low modes for when the going gets tough. However (and most importantly), the system does the thinking for you. It automatically applies as much lock up (to all the wheels) as necessary for maximum traction.