An special type of rotor-based cipher machines, are the so-called pinwheel
machines, 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 electricity for the encryption.
In most cases, the number of steps is different for each wheel – usually
a relative prime – in order to guarantee a maximum cipher period.
Note that some machines belong to more than one class.