Each subscriber had one 'vertical' of one crossbar switch on a Line Link frame permanently associated with their phone line.

All trunks -- wires leading to distant offices -- were terminated on Trunk Link frames.

The essential purpose of the whole office was to connect a subscriber's line link appearance to either another subscriber's line link appearance, if the called party was located in the same office, or to an appropriate trunk circuit on a Trunk Link frame, leading to the desired remote office.