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enter image description here

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)

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    \$\begingroup\$ The other pair can be used to carry a second phone circuit. \$\endgroup\$ – ErikR Jun 7 at 19:01
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    \$\begingroup\$ My DSL modem simply calls them Outer and Inner pair, if that's any help. It lets you select which pair to use, implying it's a fully separate line. \$\endgroup\$ – akwky Jun 8 at 8:14
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    \$\begingroup\$ I'd call them "the yellow and black wires", personally. \$\endgroup\$ – Hearth Jun 8 at 14:49
  • \$\begingroup\$ Well, way back in the '90s, I called them 'network', as those were the wires I used to network the computers in my house together using PhoneNet. \$\endgroup\$ – Glen Yates Jun 8 at 15:18
  • \$\begingroup\$ @GlenYates, would you know of any technical references re PhoneNet wiring? The Wikpedia article isn't very complete. \$\endgroup\$ – Burt_Harris Jun 8 at 23:44
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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.

enter image description here

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  • \$\begingroup\$ Yes, I know all about the RJ11 & RJ14 case, but I don't consider Wikipedia authoritative in this case. There's a lot of misinformation out there on the details. You are technically correct that black/yellow could be line 2's tip and ring in a RJ14. \$\endgroup\$ – Burt_Harris Jun 7 at 19:17
  • \$\begingroup\$ As an example of the misinformation floating around, both the diagrams you and I added are fundamentally flawed. RJ11 is a 6P4C connector, just like RJ14, everyone seems to want to leave out the outer pins. \$\endgroup\$ – Burt_Harris Jun 7 at 19:23
  • \$\begingroup\$ I've updated the diagram in my question to be more technically accurate. \$\endgroup\$ – Burt_Harris Jun 7 at 19:28
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    \$\begingroup\$ @Burt_Harris I understand you now. You should probably make that clear in your question. I don't think it's misinformation, it's simply the more modern usage of it whereas what you're really looking for is the original (historical) usage of it, right? With your clarifications, I don't think I'm qualified to correctly answer it. \$\endgroup\$ – horta Jun 7 at 20:09
  • \$\begingroup\$ Yes, Wikipedia can be incorrect. But that's why you can follow the linked references on the page to determine for yourself if reliable information is being reported. \$\endgroup\$ – chepner Jun 8 at 18:19
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UK telephone answer

If you look at the old UK telecoms specifications (BS6305 and BS6317) the main 2 wires were called A and B with "B" being pin 2 and "A" being pin 5 on the 6-pin plug. Here's a picture showing the names (A and B) on a phone that pre-dated the more modern plug/socket arrangement: -

enter image description here

Image from here.

I remembered these colour codes by thinking of the term "white bread" (my own pitiful invention) where "bread" could be shortened to B-red leaving the colour white for the A terminal. Of course, over the years the colours have somewhat changed.

As for the other two wires, I invented another simple phrase: "better to earth for safety" - this encapsulated the "B" pin being pin 2 (better to) whilst pin 4 would be earth ("for earth" becomes "four earth" (tortuous I know)). The earth wire (not a functional earth) was used to signal (Recall) to a PBX that you wanted to transfer a call.

This leaves pin 3 and, in UK phone systems this would be the shared ring capacitor used in master/slave wiring in houses (still used today) that prevents the bells tinkling on unused phones when using pulse dialling. I expect that nobody uses pulse dialling anymore.

enter image description here

Image from here.

Note also that pin 1 and pin 6 on a UK phone plug isn't normally used: -

enter image description here

  1. Not used
  2. B line
  3. Anti-tinkle wire
  4. Earth for earth recall systems (largely if not entirely not used anymore)
  5. A line
  6. Not used

And, this is a "BT" extension cord showing the old colours: -

enter image description here

Blue is the anti-tinkle wire and green is for earth recall.

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    \$\begingroup\$ Very interesting, thank you. Pins 1 and 6 aren't normally used in the US either, so much so that some people have forgotten they exist. I assume that "anti-tinkle" was something to keep the bells from ringing when another station was dialing, we had a system here to deal with that too, but I don't recall if it used an extra conductor. \$\endgroup\$ – Burt_Harris Jun 7 at 19:48
  • \$\begingroup\$ Are the modern BT connectors are physically compatible with the 6P4C connectors used in North America? Interesting the main signals to the network, A & B are on 5 & 2, while here it is always the center pair for the main line. \$\endgroup\$ – Burt_Harris Jun 7 at 19:55
  • \$\begingroup\$ @Burt_Harris I don't think they are and probably not mechanically either. \$\endgroup\$ – Andy aka Jun 7 at 19:58
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    \$\begingroup\$ Indeed they are not compatible with RJ connectors - nothing like them (e.g. the tab is on the side not the top). UK telephone connectors (BS6312 type 431A etc.) are described here en.wikipedia.org/wiki/British_telephone_socket \$\endgroup\$ – abligh Jun 8 at 5:25
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To see what went on in the Bell System in the pre-RJ era of "4-prong connectors", I looked at this document from 1972 scanned on bellsystempractices.org: Bell System Practices, Section 461-630-400

The answer is a bit underwhelming: Black is "Transformer" and yellow is "Ground and/or Transformer".

enter image description here

At least "Transformer" implies that yes AC was applied.

BTW, the document related to cable, Bell System Practices, Section 461-200-100 just calls Black/Yellow "Pair 2 Tip/Ring". (See p. 7 of the document.)

enter image description here

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  • \$\begingroup\$ Excellent, thank you. I don't consider this underwhelming in the slightest. \$\endgroup\$ – Burt_Harris Jun 7 at 20:33
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    \$\begingroup\$ By the way, the oldest reference I found, section 502-580-406, dated February 1971, and showing the 500P dial telephone wiring agrees with this (in general), and provides some more detail. It names the yellow "GRD", and shows yellow/black connecting to an (optional) transformer, without giving black a name. \$\endgroup\$ – Burt_Harris Jun 7 at 20:42
  • \$\begingroup\$ In all my years of playing with telephones, I had always assumed that black was ground. It's good to learn something new. \$\endgroup\$ – Burt_Harris Jun 7 at 20:46
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It depends. On a 2-line connection, they're line 2 tip & ring. If the cord is plugged into a 1-line jack, they're "dead" and "dead".

Note that tip & ring are electrical functions, not just wire names.

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    \$\begingroup\$ Certainly "it depends" is correct, but I think you are wrong about them being "dead" in the single line RJ11 case. The princess phone (lighted dial) and party line ringer systems made use of them. Some cheap cords had only the two center conductors, but I don't think that meet Bell System standards. \$\endgroup\$ – Burt_Harris Jun 7 at 19:08
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From Engineering and Operations in the Bell System from 1984.

In a 2-wire pair, the two leads are often called tip (T) and ring (R) after the parts of a standard telephone plug to which they connected in the days of manual switchboards. Similarly, a third wire (if present) is called sleeve. In 4-wire transmission, the four leads are called T, R, T1, and R1.

From two wire T & R, system evolved to T, R, T1, and R1 (very origional - engineers obviously). T1 & R1 were used to add features to phone.

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At one point the black and yellow were used for "Princess" line phones, I believe to at least light up the dial. I say this because when a Bell lineman came to add extensions (Upstate NY USA, 70's) asked - "Do you have any princess line telephones?" when asked about it, replied - "Have to know as the black and yellow were used for them, affects how to wire if in use." interested - search "bell princess line telephone"

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  • \$\begingroup\$ Yes, that's true, but it doesn't address the question about names for them. \$\endgroup\$ – Burt_Harris Jun 8 at 23:41
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Answering my own question. All the other asnswers are certainly valid, and the only universal answer is "it depends." I really wanted to see if anyone else would come up with the designations "A" and "A1" for black and yellow respectively. I think these were the prevalent terminology inside the Bell System since the mid-late 1970s, at least when it wasn't a two-line setup.

Here's the wiring diagram of the Model 2500SM taken from the Bell System practices, The 2500SM was the first generally available phone that had the modular connectors in RJ11 form. The full document shows the before and after diagrams. As wired from the factory, BK and Y were A and A1.

Wiring diagram of Western Electric model 2500

And I have a follow-on question for any of you with an ohmmeter and POTS telephone: Are black and yellow leads from your phone shorted (near 0 ohms) when pick-up the handset? If so, your phone is set up for RJ11 and not RJ14. (You can't assume something is RJ14 just because it has 4 wires. In my experience, true RJ14s are very rare.)

I remember from my youth the names A and A1 being used in a set of old Key Telephone System manuals, but they had different color code (and a lot more wires) in a key set. I really hadn't considered them as designations for yellow and black till recently. I believe A stands for Auxillary and is based on a definition in FCC regulations.

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  • \$\begingroup\$ Yes, (Bell/WE) 1A2 key systems used 3 pairs per line: T,R, A,A1, L,L1; often as plastic-jacketed cables of 6, 9, 12, or 15 twisted pairs using the two-color coding (white-with-blue and blue-with-white; white-with-orange and orange-with-white; etc). If you wanted to put a single-line set on a key system with restricted functionality, yes you could use quad (which wasn't twisted, at least not always) with A,A1 on Y&B. \$\endgroup\$ – dave_thompson_085 Jun 9 at 0:37
  • \$\begingroup\$ @dave_thompson_085, you know your stuff, but the 1A2 I took apart repeatedly used a 25 pair connection. I think the L/L1 pair were for the per-line lights, right? \$\endgroup\$ – Burt_Harris Jun 9 at 0:42
  • \$\begingroup\$ The 25-pair was officially an RJ21. tripplite.com/… \$\endgroup\$ – Burt_Harris Jun 9 at 0:57
  • \$\begingroup\$ L,L1 were lamp, yes. Some key sets (I don't remember exactly which) used 25-pair, which 'wasted' a little capacity but was easy to stock in bulk. And if a line appearance went through multiple closets or cabinets (e.g. in a large office building with multiple floors) the 'internal' connections could use available pairs on any building cable -- which were often 100-pair or more. \$\endgroup\$ – dave_thompson_085 Jun 10 at 2:31

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