A twin-clutch gearbox or dual clutch transmission (DCT) is a semi-automatic transmission with separate clutches for odd and even gears. The outer clutch drives the odd numbered gears and reverse, while the inner clutch drives the even numbered gears. Shifts can be accomplished without interrupting power, by applying the engine's torque to one clutch just as the engine's torque is being disconnected from the other clutch. Since the synchronizers that select an odd gear can be moved while driving the car in an even gear, and vice versa, DCT's have been configured which shift faster than Formula One cars and other single-clutch AMT's (automated-manual transmissions, a.k.a. single-clutch semi-automatics); the shift can also be made smoother and more suitable for street-driving than a single-clutch AMT is capable of.
A twin-clutch gearbox eliminates the torque converter used in traditional automatic transmissions. However, dual clutch transmissions that are currently on the market use wet multi-plate clutches, similar to the clutches used in traditional automatic transmissions. Versions that use dry clutches, like those usually associated with manual transmissions, are rumored to be in development by several manufacturers.
This type of gearbox was invented by Andolphe Kégresse just before the outbreak of World War II, but he never developed a working model. The first actual DCT's came along in the 1980s when computers to control the shifts were compact enough: the PDK (Porsche Doppelkupplung) used in the Porsche 956 and 962 Le Mans race cars and the Sport Quattro S1 race car.
DCTqs are currently sold mostly by Volkswagen Group under the brand names DSG Direct-Shift Gearbox (for Volkswagen, SEAT and Škoda) and S-Tronic (for Audi). Volkswagen also produces a DCT transmission (model DQ250) under license from Borg-Warner for use in transverse powertrain installations, and this is used in the Golf/Rabbit, Jetta, Passat, Touran, Audi A3 and Skoda Octavia. Volkswagen are believed to be developing a family of DCTs to go across the VAG product range. UK-based Ricardo Company produces the seven-speed DCT for the 1001-horsepower Bugatti Veyron 16.4, another Volkswagen Group product.
Getrag has a complete range of DCT transaxles, including 7DCL750, a seven-speed transaxle for mid-engine longitudinal applications, capable of taking more than 750 N·m; it is "in development" with a release expected by early 2007. Getrag and Chrysler will jointly build a plant in Indiana that will supply dual-clutch transmissions to Chrysler starting in 2009.
A renewed version of the PDK (Porsche Doppelkupplung), but for mass-production instead of race cars, was expected in January 2006 but may come in 2007 or later. Porsche is said to have filed paperwork indicating that it will be available as a late addition to the 997, in future model-years. 
BorgWarner is the current leading supplier of wet clutches and hydraulic modules for these transmissions. BorgWarner, who call their technology "DualTronic", signed further agreements with two other (unnamed) European automotive manufacturers to incorporate their components in DCTs.