This is the simplest type of gearbox. 

Figure gives a simplified view of the gearbox. 

The power comes from the engine to the clutch shaft and thence to the clutch gear which is always in mesh with the gear on the layshaft. 

All the gears on the layshaft are fixed to it and as such they are all the time rotating when the engine is running and the clutch is engaged. 

Three direct and one reverse speeds are attained on suitably moving the gear on the main shaft by means of selector mechanism.