A line that directly interconnects, or ties together, two key telephone systems (KTSs) in a private network configuration, a tie line is a dedicated circuit that generally is leased from a public carrier. A tie line allows users to avoid toll charges for long-distance calls between two geographically separated offices. If a key system sits behind (i.e., is used in conjunction with) a PBX, one or more tie lines are used to interconnect them. See also line, nailed-up circuit, and tie trunk.