Sequential transmissions are commonly found on motorbikes, where the action of selecting gear is limited to the next higher or lower gear; selecting a specific gear, as is possible in an H-pattern gearbox, is not possible in a sequential transmission.
Sequential transmissions use dog engagement for ultra fast gear changes where a synchromesh engagement would not be fast enough. This allows gear changes to be made with very brief interruption of torque from the engine, which means changes can be made without the need to use the clutch.
Sequential gearboxes should not be confused with DSG style gearboxes that are fitted to many new road cars. A DSG is still a manual, synchromesh gearbox, or technically two manual gearboxes in one, and uses two clutches to achieve much faster shift times by pre-selecting the next gear. DSG transmissions are typically heavier than a true sequential gearbox due to this though.
Quaife Engineering offers a range of complete universal sequential transmissions, as well as numerous vehicle specific sequential gearboxes and gearkits.