The basic module comprises a gearbox with four gears. By adding a front- mounted and a rear-mounted module, a sixteen-speed gearbox results. With the help of the left-hand shifting sleeve, the counter shaft can be driven with two transmission ratios. In this way, eight forward gears result from four gears. In the rear-mounted group, two transmission ratios of the secondary shaft can be shifted with the help of a simple planetary gear set. Eight gear steps times two results in sixteen speeds.