From the document DE 103 05 241 A1, a six-gear or seven-gear dual-clutch transmission is known. The dual-clutch transmission has two clutches, each connected on its input side to the drive input shaft and, on its output side, to one of the two respective transmission input shafts. The two transmission input shafts are arranged coaxially with one another. Further, two countershafts are arranged axis-parallel to the two transmission input shafts, whose idler gearwheels mesh with fixed gearwheels of the transmission input shafts. Moreover, coupling devices are held rotationally fixed but able to move axially, on the countershafts, in order to be able to shift the respective gearwheels of the gears. The transmission ratio in each case is transmitted by the drive output gearwheels to a differential gearbox. To produce the desired transmission ratio steps with the known dual-clutch transmissions, a substantial number of wheel planes are required, so that the structural space required for installing them is not inconsiderable.
Further, from the document DE 38 22 330 A1 a spur gear variable-speed transmission is known. The spur gear variable-speed transmission comprises a dual clutch that can be shifted under load, one part of which is connected to a drive input shaft and whose other part is connected with a hollow drive input shaft mounted to rotate on the drive input shaft. For certain transmission ratios, the drive input shaft can be coupled to the hollow drive input shaft by means of a shifting element.
From the document DE 10 2004 001 961 A1 a shift-under-load transmission with two clutches is known, each of which is associated with a part-transmission. The transmission input shafts of the two part-transmissions are arranged coaxially with one another and are engaged, via fixed gearwheels, with idler gearwheels of the associated countershafts. The respective idler gearwheels of the countershafts can be connected rotationally fixed to their countershafts by means of associated shifting elements. From that document, a six-gear transmission is known in which a further shifting element is provided for coupling the two transmission input shafts in order to create a further transmission ratio step. In this design version, the six-gear transmission requires at least five wheel planes in the two part-transmissions in order to be able to produce the transmission ratio steps. This results in an undesired increase of the structural length, in the axial direction, which considerably restricts the possibility of installation in a vehicle.