Input: green internal gear.
Output: blue internal gear or vice versa.
The internal gears have the same gear number: Ni
Orange and grey external gears have the same gear number: Ne
No compulsory relation between Ni and Ne. Here Ni = 2.Ne
The green gear is in mesh with the orange gear.
The blue gear is in mesh with the grey gear.
The orange gear is in mesh with the grey gear.
The coaxial input and output rotate in opposite directions with the same velocity.