Doing some work on old style land line phone system?

Question

For my parents everything seems to run on the red and green wires ,but can anyone tell me what the yellow and black pair are for ? I thought at first one might be a ground,but a seperate pair?could they be reserved for a second line,? Thanks Rich

Answer 1

the green/red pair is for voice, the black/yellow pair can be used for line 2, but in the OLD DAYS, and the Princess Phone was the rage, the black/yellow pair was the battery supply for the lights in the phone.

Answer 2

commonly used for a second line.

Answer 3

Yep, second line it is!  My husband has been a phone tech for years and that's what he said too.

Answer 4

These days on regular home wiring the yellow/black pair is most often used for a second line, but there are other applications.  For example, if you had a DSL filter fitted at your network interface you might then use the red/green pair for the filtered line to voice jacks and the yellow/black pair as the unfiltered line for your DSL modem (or vice versa).

Other uses were quite common in the past.   Remember the Princess phones which had a dial light?   The yellow/black pair was used to provide power for those lights from a transformer which could be hidden away in a closet or some other convenient location.

Many phones on party lines in the past also needed a local ground connection for the bell, and this was usually done via the yellow wire.  In fact at one time the old 500 rotary phones were fitted with a 3-core cord which just had red, green, and yellow wires, no black.