Also known as a backbone switch and a core switch, a tandem switch is a high-capacity switch positioned in the physical core, or backbone, of a Public Switched Telephone Network (PSTN), where it serves to interconnect edge switches, or Central Office (CO) switches. In the traditional PSTN hierarchy, a tandem might be a Class 1 regional toll center, Class 2 sectional toll center, Class 3 primary toll center, or Class 4 tandem toll center. An access tandem switch serves to connect local exchange carriers (LECs), i.e., local telephone companies, to the interexchange carriers (IXCs), i.e., long distance carriers, over dedicated interoffice trunks, known as access trunks. In a contemporary PSTN, a tandem switch commonly is a hybrid Class 4/5, functioning as both a tandem and a CO (Class 5). See also IXC, LEC, PSTN, and switch.