A two-motion selector can therefore accept two dialled digits from a subscriber and route the call to any of 100 numbers. The selector 'wipers' always start in their resting 'home' position. The first digit moves the selector veritically up to the corresponding level and then the second digit moves the wipers around the contacts of that level. This is shown in figure 3, below. The type of selector shown above is known as a Final Selector as it takes the final two digits of the number dialled.
Most numbers dialled are several digits longer, and therefore pass through a chain of selectors.
Group selectors take only ONE digit from the caller, and step up the number of levels according to the digit dialled. This is covered in more depth later. In Strowger's system, selecting digits to dial was done by a complicated system involving five separate wires. With TP dialling, only one pair of wires is required for a telephone, the speech pair. To dial a digit, the circuit is interrupted according to the number dialled so, for example, if you dialled a '4' then the line would be pulsed four times, quickly in succession.