the differential lock. The transfer case may contain one or more sets of low range gears for off-road use. Low range gears are engaged with a shifter or electronic switch. In many transfer cases, this shifter is the same as the one that selects 2WD or 4WD operation. Low range gears allow the vehicle to drive at much slower speeds while still operating within the usable power band / RPM range of the engine. This also increases the torque available at the axles. Low-range gears are used for very inclement road conditions, towing a heavy load slowly, driving on rough, unimproved roads slowly, and extreme off-road maneuvers such as rockcrawling. This feature is often absent on all-wheel-drive cars. Some very large vehicles, such as heavy equipment or military trucks, may have more than one low-range gear. Types Transfer cases used on "part-time" four-wheel-drive off-road vehicles such as trucks, truggies, rock-crawling vehicles and some military vehicles generally allow the driver to select 2WD or 4WD, as well as high or low gear ranges. Those used in sports cars and performance sedans are usually "transparent" to the driver; there is no shifter or select lever. Drive types Gear-driven There are two different types of internal power-transfer mechanisms found in most transfer cases. Gear-driven transfer cases use sets of gears to drive either the front or both the front and rear driveshafts. These are generally strong, heavy units that are used in large trucks, but there are currently several gear drive cases in production for passenger cars. Chain-driven Chain-driven transfer cases use a chain to drive most often only one axle but can drive both axles. Chain-driven transfer cases are quieter and lighter than gear-driven ones. They are used in vehicles such as compact trucks, full-size trucks, Jeeps, and SUVs. Some off-road driving enthusiasts modify their vehicles to use gear-driven transfer cases, accepting the additional weight and noise to gain the extra strength they generally provide. Housing type Married Transfer cases are also classified as either "divorced"/independent or "married". Married transfer cases are bolted directly to the transmission, usually between the transmission's output shaft and the rear or main driveshaft. Sometimes a married transfer case is an integral part of the transmission and the two components share the same housing or "case", as is commonly found on recent Subaru products and some other all-wheel-drive cars. Divorced/independent A divorced or independent transfer case is