Tip and ring
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"Tip" and "ring" are commonly known terms in the telephone service industry referring to the two sides of an ordinary telephone line corresponding, respectively, to the ground side and battery side of a (DC) phone circuit. (Such circuits are known as POTS lines, which stands for Plain Old Telephone Service). The ground side is common with the telco's central office ground; the battery side carries approximately 50 volts of DC current when in an "idle" or "on hook" state. The combination of tip and ring, then, makes up a normal phone line circuit, just as your car's battery needs both POSITIVE and GROUND/NEGATIVE leads to have a complete electrical system. When the phone rings, about 90 volts of AC current is superimposed over the DC current already present on the idle line.
Some years ago, tip and ring had to be in the correct order at any given phone jack in order to be able to make outgoing calls with a Touch Tone phone. If they were reversed in polarity, there would still be a dialtone and calls could be received, but outgoing calls would not be possible: the Touch Tone sounds would never "break" the dialtone and nothing would happen. But in most phones manufactured in the last few decades, "polarity guards" made that problem disappear. Today, tip and ring polarity is mostly immaterial, except for special kinds of phone circuits like DID (Direct Inward Dialing) trunks, T-1s and ground start PBX lines where the field side ("terminal") equipment--like a company's PBX switch--needs tip and ring polarity to be correct before they can function.
