For this reason the line finder had, besides the tip and ring leads for talking and the sleeve lead for control, a fourth wire for the District Circuit to send metering pulses to control the message register.

For outgoing calls, the Line Link Frame acted like the Line Finder of the panel switch, autonomously connecting the line to a junctor, which corresponded to the cord circuit of the old cord telephone switchboard.

These switches had some design similarities to No. 1 step-by-step systems already in use in large offices (10000 subscribers or more) but were of a simpler design (line finders would only need to scan hundreds of lines instead of the thousands in use in the cities) and not designed for expandability.