Transmission Designs
Mechanical stepped transmissions are distinguished into two designs:
Dual shaft transmission
Epicyclic transmission (planetary gearbox)

Dual Shaft Transmission
The dual shaft transmission, also called countershaft transmission, is the generic term applicable to all conventional stepped transmissions. In this case, the transfer of torque in the non-direct gear levels takes place over externally toothed spur gear pairs mounted on two parallel shafts.
The spur gear pairs for the forward gears are permanently meshed with each other. They consist of a fixed gear wheel connected to a shaft and a free gear wheel. The latter is positioned on the second shaft and can freely rotated but cannot be axially displaced. Positively-engaging clutch elements are used to engage the freely rotating gears to the shaft, as per demand.
Dual shaft transmissions can be classified according to the position of the transmission input and output shafts into:
Coaxial two-shaft transmission
Deaxial two-shaft transmission

Coaxial Dual-Shaft Transmission
In the case of standard-driven vehicles (front engine with rear wheel drive), the dual-shaft mechanical stepped transmission in coaxial design is predominantly used. The coaxial position of input and output shaft which can normally be connected rigidly in the direct gear, is characteristic to this design. The power flow for the remaining gears is led over a countershaft.
As a rule, the coaxial countershaft transmission is directly coupled with the engine along with the clutch assembly. The output is led over the cardan shaft. This leads to an unfavorable axle load distribution. This is the reason behind the transmission being located at the rear axle in some vehicles (transaxle design). The cardan shaft then rotates at the engine speed.