With neat diagrams explains the configuration of a step-by-step switching system.

The schematic diagram for an exchange is specified in figure. Each subscriber is associated to a single rotary pre-selector switch on the exchange, the outputs from such switch being linked to a bank of two-motion switches termed as 'group selectors'. The out-puts  from  the  preselected  switches of an entire  group  of subscribers are connected together into parallel as such group of subscribers share a particular bank of group selectors.  While a subscriber lifts his telephone, the cradle switch causes a circuit to be finished back to the telephone exchange, signaling which the subscriber wishes to make a call. It causes the pre-selector switch to step around till this finds a free group selector. The pre-selector switch after that stop in this position and the group selector is 'seized' through the subscriber wishing to give a call.

FIG - 1000 Line Exchange

Upon seizing the group selector, the subscriber dials his primary digit and the selector switch moves up to the suitable row on the switch contact array. Each last selector has the chance of connection to 100 lines. Therefore the 1000 lines are divided in 10 groups of 100 each, the group being specified by the first digit into the subscriber's number. The vertical motion of the group selector hence selects a last selector in the group related with the first digit dialled. All individual rows of contacts or levels, of the group selector are associated to a bank of last selectors related with a particular group of 100 line numbers. Contain dialled the first digit to choose the suitable group, the group selector arm after that automatically rotates in the vertical direction till it finds a free final selector. In the last selector, both directions of motion are in the control of the subscriber's dial and, after the dialling two further digits, link is established, giving the called subscriber's to answer his telephone.