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  • Older two conductor: Red is negative, black is positive.
  • Older three conductor: Positive was red, negative was black and earth was yellow. The addition of the earth ground allowed for the use of party lines.
  • Four conductor: Positive was red, negative was black. The second pair of wires was connected to a transformer creating 48VDC to power the lights on telephone sets like the Princess model. Black is positive and yellow is negative. When LEDs allowed phone illumination to be powered with the Central Office battery and ground, the second set of wires could be used for a second line.
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