Where large numbers of gear ratios, six or more, are to be provided then these will require a third or more gear clusters on the layshaft. To maintain the proportions of the overall gearbox as more compact, rather than becoming long and thin, these gearboxes may use twin layshafts. This requires an additional driven gear for each layshaft, but the mechanism is otherwise very similar. The use of multiple layshafts also developed into the multiple clutch gearbox, used for some buses, where each ratio has its own layshaft and separate plate or hydraulic clutches, rather than dog clutches, are used to select between them.
