An alternative class of rotor-based cipher machines, is the so-called pin-wheel
machine, in which a complex system of movable pins controls the
stepping of each wheel. Most pin-wheel machines are purely mechanical,
which means that they do not require electric energy for the encryption.
In most cases, the number of steps is different for each wheel – commonly
a relative prime – in order to guarantee a maximum cipher period.
Note that some machines belong to more than one class.