Tandem switch

In this image, offices A, X, Y, and Z are local central offices, each serving telephone subscribers. Office "P" is a tandem office, which interconnects local offices A, X, Y, and Z. Note that the local offices can also have direct paths to each other, if necessary.

A tandem switch (or simply a tandem) is a type of telephone switch that connects exchanges to other exchanges. This is in contrast to a traditional class 5 exchange that connects subscribers to other subscribers. In a tandem office, there are no subscribers per se, since all of the incoming and outgoing paths to a tandem are connected to other telephone switches in the nearby network.