all about phone plugs

all about phone plugs
The little plastic tips on the ends of phone cords and cables are modular plugs. They fit into modular jacks. Despite their male name, jacks are female. Plugs are male

The word “modular” refers to a phone manufacturing format introduced by AT&T in the 1970s which allowed installers to assemble phones at a customer’s location by selecting specific components that plugged together, instead of needing "hard wiring" with screwdrivers at the factory. This made it easier to replace defective equipment and provided more phone choices with less inventory in the trucks. For example, if an installer had rotary-dial and touch-tone Trimline handsets, and both desk and wall bases, the four phone halves could be assembled to make four different types of complete phones. Six cord lengths provided even more options.

The modular connector design was also applied to the jacks on walls that phones were plugged into.

Modular plugs are fragile. Repeated plugging and unplugging will cause the tabs to snap off. So will pulling a cord through a snarled mess of wires. If you’re the phone person for your business or home, invest in a supply of plugs and a plug crimper to attach them. If you frequently have to pull cords through tight quarters, get cords with "boots" to protect the tabs.

Modular plugs are made in three basic sizes:

  • The smallest plug, known as 4-position, 2-conductor (or 2 wire), is used for handset cords. A "position" is a groove molded into the plastic that could contain a little bit of gold-plated wire to make contact with wires inside the jack.
  • The middle-size plug is the most common. It has six positions, and two, four, or six wires. It is used for most line cords that connect phones, modems and other devices to phone jacks.
  • The largest common plug, with eight positions and eight wires, is usually used for LANs (Local Area Networks) and sometimes for four-line phones. It is often called an RJ-45, but that designation is inaccurate for LANs.
  • Eight-wire plugs and jacks are also used on some ATT, Lucent and Avaya phone systems. If you are going to re-use jacks previously installed for a Merlin or other phone system that uses the “T568B” wiring scheme, you will either have to re-arrange the wires inside the jack, or connect the circuit that would normally go on the white/orange wire pair, to the white/green pair.

There are other variations that you should be aware of, mostly so you don’t get the wrong thing:
  • Each size plug is made in versions for both solid and stranded wire. Solid wire usually goes inside and on walls. Stranded wire is used for the cords that go from phones to jacks, and for patch cords used for patch panels.
  • There are 6-pin modular plugs with the locking tab at the end instead of the middle, but still on the bottom, made for use with certain products From Digital Equipment Corporation (DEC).
  • In the United Kingdom, the tabs are on the ends of the plugs, not on the bottom.
  • There are also 8-pin “keyed” plugs with an extra bit of plastic on one end to keep them from being inserted into standard jacks.
  • There’s a 10-pin modular plug with an “unofficial” RJ-50 designation used for some special applications. It’s not used for phones.

At least 99% of all phone jacks used with one-line phones could be two-line jacks. In the same jack, if two wires are connected, it's a one-line jack. If four wires are connected, it's a two-liner. (This explanation applies to phone jacks connected directly to phone company dial tone, NOT to jacks within a phone system.)

The FCC and phone companies have codes to identify how jacks are wired. Registered Jack numbers end with a letter indicating the wiring or mounting method being used.

“RJ” stands for Registered Jack. A single-line jack designed for mounting a wall phone is an RJ-11W. A single-line jack for a table or desk phone is an RJ-11C. Two-line jacks are RJ-14W and RJ-14C. Three-line jacks are RJ-25. If you know that much, you know more than many phone company employees. Four-line jacks are RJ-61. If you know that, you know more than 97% of phone company employees.

“W” stands for Wall. “C” identifies a surface or flush-mounted jack. Apparently only one person knew what the “C” actually stood for, and she died without telling anyone. “S” identifies a single-line jack. “M” identifies a multi-line jack. “X” identifies a complex multi-line or series-type jack. Series jacks are used for alarm dialers and other devices.


RJ-11C or W

One phone line

RJ-14C or W

Two phone lines


Up to 25 phone lines

RJ-25C or W

Three phone lines


For alarm dialers

RJ-28C/J & RJ-25S

Four wire data circuits

RJ-61C or W

Four phone lines


These “RJ” names do not really refer to official RJ types.
In the chart, “P” means “position,” and “C” means “conductor” (wire).

“RJ9,” “RJ10,” “RJ22”

2P2C or 2P2C, for telephone handsets. Since telephone handsets do not connect directly to the public telecom network, they have no Registered Jack code.


8P8C, informal designation for T568A/T568B, including Ethernet; not the same as the true RJ-25 and RJ-25S


10P10C, for data

Lots of people confuse simple self-contained multi-line phones with the phones used in systems. Multi-line phones used in modern electronic or digital phone systems usually have cords with two or four wires (one or two pairs of wire), regardless of the number of lines on the phone. With today’s technology, one simple pair can handle dozens of phone lines.

If you look at the springy wires inside an ordinary phone jack (one that’s NOT used with a phone system), the two inner wires are used for line #1, and the two outer ones are used for line #2. The flat cords that commonly connect phones to phone jacks follow this same arrangement.

Cords used for three-line phones have two additional conductors for the third line, outside the second pair, for a total of six wires (three pairs). If you look at a cross-section of a six-conductor phone cord, the line circuits could be considered to look like this: 321123. If you plug a single-line phone into a two-line jack, it will work on line #1. If you plug a two-line phone into a three-line jack, it will work on line #1 and line #2.

Four-line cords have two more wires (total of eight wires, in four pairs). Four-line non-system phones can use two two-line cords, or one four-line cord, depending on the phone designer’s preference. Most use two cords.

The diagrams show how jack pins are numbered. By the way, a jack should be installed so the slot for the plug’s tab is at the bottom, and the wire springs are at the top. This way, if any liquid drips into the jack, it won’t cause the pins to short-circuit or corrode. The diagrams were provided by Siemon. We thank them.