TRS connector
The cylindrical plug with a tip, a ring and a sleeve — inherited from 19th-century telephone switchboards and still everywhere in audio.
The American name, phone connector, gives the origin away: operators patched calls with quarter-inch plugs on manual exchanges. Each contact surface is a conductor — tip, ring, sleeve — so the same plug does entirely different jobs. As a balanced mono line, hot rides on the tip and cold on the ring. In headphones, tip and ring carry left and right. In an insert cable, the signal goes out on one and comes back on the other.
The 3.5 mm miniature version arrived with the transistor radio and became standard in everything portable; TRRS adds a ring for a microphone. Unlike XLR, a TRS plug shorts its contacts as it slides in, so don't plug it in with the monitors up.