Tip and ring

Tip and ring are the two conductors or sides of a telephone line. Their names are derived from the telephone plugs used for connecting telephone calls in manual switchboards. One side of the line is connected to the metal tip of the plug, and the second is connected to a metal ring behind the tip, separated and insulated from the tip by a non-conducting material. When inserted into a jack, the plug's tip conductor connects first, followed by the ring conductor. In many European countries, tip and ring are referred to as the A and B wires.

Neither of the tip and ring conductors is permanently connected to earth ground, but may be connected to ground during signaling operations. Typically, the ring conductor has a direct current (DC) potential of −48V to −52V with respect to the tip conductor when the line is in the on-hook (idle) state. Floating both conductors, not referencing either one to ground, minimizes the pickup of hum from any nearby alternating current (AC) power wires.