In telephone company (telco) terminology, line describes the user side or local loop side of the central office (CO).The customer-facing side of the public switched telephone network (PSTN). In other words, the line side is the side of the network to which users connect in order to access the network core, or backbone.The line side is synonymous with the local loop access circuits, with the demarcation point being the local loop interface at the CO. The trunk side involves the high-capacity trunks that serve to interconnect the various telco switching centers in the core of the carrier network. See also CO, line, local loop, PSTN, and trunk side.