"Vinnie" wrote in message
.. .

The yellow wire in older phone wiring is part of the yellow/black pair.
They work just like the red/green paid (two conductors required for one
phone line), but the yellow/black pair are normally reserved for the
second line in the house. Red/green are normally used for the first
line. For a second line, the yellow wire goes to the red terminal on
the jack, and the black wire goes to the green terminal on the jack.

Back in the 60's (?) the yellow/black pair were often used for the 9 volt
power supply from a wall mounted transformer to operate the light in
'Princess phone' dials.