Gear boxes are used to get variable output torque as per requirement. The torque is increased by transferring power from a smaller gear to a larger gear and torque is decreased (rpm in increased) and vice versa.

In general, two-stage gearbox means in which the torque is stepped down or up twice that is using two pairs of gears as shown in the figure below.

In the gear box if 'n' number of stages, $i_0$ is overall Gear Ratio, Then -

$$ i_{stage} = n_\sqrt{i_0}, \text{ using geometric progression (GP)} $$

In two stage gear box, $ i_1 = i_2 = i_{stage} = 2_\sqrt{i_0}, \quad i_0 = \frac{N_1}{N_2} \times \frac{N_3}{N_4} $

The figure below is worm gear box used for higher transmission range.