A gearbox configured for use in a vehicle comprising a first power source that is configured to rotate in a first rotational direction and a second power source that is configured to rotate in both the first rotational direction and a second rotational direction opposite to the first rotational direction, the gearbox comprising: a first main clutch; a second main clutch which is actuatable independently of the first main clutch; an input shaft for connection to the first and second power sources; an outer intermediate shaft removably coupled to the input shaft by the second main clutch, and carrying a first set of shaft gears; an inner intermediate shaft removably coupled to the input shaft by the first main clutch, and carrying a second set of shaft gears, the inner intermediate shaft running concentrically within the outer intermediate shaft; a first lay shaft carrying a first set of drive gears and a first output gear, a second lay shaft carrying a second set of drive gears and a second output gear, each drive gear being coupled to a respective shaft gear to together provide a plurality of gear ratios between the intermediate shafts and the output shaft; a plurality of coupling mechanisms (60;61;62), wherein at least two drive gears on a lay shaft share a common coupling mechanism for selectively coupling the two drive gears to the layshaft; wherein the first set of shaft gears are coupled to respective drive gears to provide a first set of gear ratios of the plurality of gear ratios, and the second set of shaft gears are coupled to respective drive gears to provide a second set of gear ratios of the plurality of gear ratios, each gear ratio of the first and second sets of gear ratios being a forward gear ratio; characterised in that the gearbox further comprises an output shaft configured to be coupled to the wheels of a vehicle by a differential, each lay shaft being coupled to the output shaft by a respective one of the first and second output gears, and the first output gear is positioned along the first lay shaft between two of the first set of drive gears and the second output gear is positioned along the second lay shaft between two of the second set of drive gears. a first main clutch; a second main clutch which is actuatable independently of the first main clutch; an input shaft for connection to the first and second power sources; an outer intermediate shaft removably coupled to the input shaft by the second main clutch, and carrying a first set of shaft gears; an inner intermediate shaft removably coupled to the input shaft by the first main clutch, and carrying a second set of shaft gears, the inner intermediate shaft running concentrically within the outer intermediate shaft; a first lay shaft carrying a first set of drive gears and a first output gear, a second lay shaft carrying a second set of drive gears and a second output gear, each drive gear being coupled to a respective shaft gear to together provide a plurality of gear ratios between the intermediate shafts and the output shaft; a plurality of coupling mechanisms (60;61;62), wherein at least two drive gears on a lay shaft share a common coupling mechanism for selectively coupling the two drive gears to the layshaft; wherein the first set of shaft gears are coupled to respective drive gears to provide a first set of gear ratios of the plurality of gear ratios, and the second set of shaft gears are coupled to respective drive gears to provide a second set of gear ratios of the plurality of gear ratios, each gear ratio of the first and second sets of gear ratios being a forward gear ratio; characterised in that the gearbox further comprises an output shaft configured to be coupled to the wheels of a vehicle by a differential, each lay shaft being coupled to the output shaft by a respective one of the first and second output gears, and the first output gear is positioned along the first lay shaft between two of the first set of drive gears and the second output gear is positioned along the second lay shaft between two of the second set of drive gears.