When Touch-Tone service was introduced in the 1960s, the dual-tone multi-frequency signaling (DTMF) tone generator also required correct polarity as it depended on the line D.C. voltage for operation. Later Touch-Tone telephones included a diode bridge that eliminated the polarity sensitivity so that consumer telephone service is essentially immune to reversal today. However, some special circuits, such as some direct inward dialing (DID) trunks, T-1 lines, and ground start lines connected to field side ("terminal") equipment, e.g., a corporate private branch exchange (PBX) switch, correctly operate only with proper tip and ring polarity.