Invented by a Frenchman just before WW2 and introduced to road cars in 2003 in the Golf R32, this type of transmission is a semi-automatic gearbox - two clutches take care of two sets of gears.
One clutch takes care of odd gears (
1, 3, 5, 7) and the other
clutch deals with the even gears (R, 2, 4, 6).
How it works:
One clutch has 1st gear engaged, with the car moving forwards. Meanwhile, the other clutch has 2nd gear ready to be engaged. Come change-up time, clutch no.
1 disengages 1st and clutch no.2 engages 2nd gear and the car keeps going, whilst clutch no.
1 prepares 3rd gear for engagement. All of this happens within milliseconds, so power delivery to the road is uninterrupted.
Usually - though not always - dual-clutch gearboxes are more fuel-efficient than an equivalent manual gearbox.
For more info, see
Wikipedia.