Any wannabe telco guy can tell you that the green and red pair are called "tip" and "ring" respectively, most can tell you the historical reason why. That's not what I'm looking for.
As a baby boomer, I learned early that the yellow and black wires usually didn't do a thing. I know there are exceptions, for example lighted phones and party lines, but what names are used for these conductors?
There's no single right answer. I'm particularly looking for Bell System terminology, but I'm interested in any designation you can cite technical documentation for.
As in the 1970s, I want to yellow/black for power. Which polarity would you use?
Was AC ever applied to yellow/black, e.g. in the princess phones?
As much of the discussion on this question has focused on registered jacks, and misinformation on the web, I thought I'd dig up the legally binding definitions, in FCC Rule 69.500. That isn't easy reading, but really only RJ11 and RJ14 are relevant. I'll copy those diagrams and attach inline above. (to do)
Best Answer
This is going to be boring but from what multiple sources are saying, they're called, wait for it.... Tip2 and Ring2. The green and red are Tip1 and Ring1.
The jack you speak of is RJ14 (vs the RJ11 which is the 2 wire jack but the same size).
It appears they're simply used to have more lines to a single phone or they're used for junction boxes so you can have two lines that are eventually split later according to this. Effectively they're no different than the original pair.
The wikipedia page gives you more details.