about phone jacks & plugs

The word "modular" refers to a phone construction format introduced by AT&T in the1970s, that allowed installers to assemble phones at customer locations by selecting specific components that plugged together, instead of needing hard wiring. The modular connector design was also applied to the jacks that phones plugged into, to get dialtone.

Modular plugs are made in three sizes:

• The smallest plug, known as 4-position, 4-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 either two, four, or six wires. It is used for most line cords that connect phones, modems and other devices to phone jacks.
• The largest 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 RJ45, but that designation is inaccurate for LAN connections. 
• Eight-wire plugs and jacks are also used on some ATT/Lucent phone systems. If you are going to re-use jacks previously installed for a Merlin or other ATT / Lucent / Avaya 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.

At least 99% of all phone jacks used with one-line phones are really two-line jacks. In the same actual jack, if two wires are connected, it's a one-line jack. If four wires are connected, it's a two-liner.

The FCC and phone companies have codes to identify how jacks are wired. 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 all phone company employees. "RJ" stands for Registered Jack. "W" stands for Wall. Apparently only one person knew what the "C" stood for, and he died without telling anyone.

If you look at the springy wires inside a common phone jack, the two inner ones 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 lines #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.

Multi-line phones used in modern electronic or digital phone systems usually use cords with four or six internal wires, regardless of the number of lines the phone can handle.

Phone jacks are made in three general formats:

The surface jack, is a cube, roughly 2 inches by two inches by 3/4 of an inch, and mounts on the surface of the wall. It is usually installed after a wall is constructed, often with wire stapled to the baseboard. They are available in a variety of colors, with connections for two, three or four pairs.

The flush jack is flat, like an electrical outlet, and is usually installed in new construction, with wire concealed inside the wall. Flush jacks are available in a variety of colors, for two, three or four pairs of wire, and in both standard and "Decora" styles (at right, above).

The wall jack, usually mounted about five feet above the floor, has two metal mounting studs, to support a wall phone. They are available with plastic or stainless steel front plates, with two or three pairs of wire.

Both  surface and flush jacks are available in multi-port versions, with up to six ports in a single-gang plate or12 ports in double-gang plate. You can use a mixture of jacks and other connectors, for phone, fax, LAN, modem, answerer, TV, etc. Multi-port assemblies are available in both standard and Decora styles, in a variety of colors.

Special purpose jacks and mounting plates are available for installation in the floor, outdoors, in modular furniture,  etc.