A trunk that directly interconnects, or ties together, two PBXs in a private network configuration, a tie trunk is a dedicated circuit that generally is leased from a public carrier. Through the use of optional automatic route selection (ARS) software, PBX systems can automatically route calls between offices over an available tie trunk rather than over the public switched telephone network (PSTN), thereby avoiding toll charges. See also nailed-up circuit, tie line, and trunk.