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(hardware, communications)
An American-style telephone jack with six possible connections. A telephone normally uses two pairs of wires. Often found on the back of US-manufactured modems or for connection to a leased line.
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(Registered Jack-11) A telephone interface that uses a cable of twisted wire pairs and a modular jack with two, four or six contacts. RJ-11 is the common connector for plugging a telephone into the wall and the handset into the telephone. See modular connector.

RJ-11, 14, 25 - One, Two and Three Lines
RJ-11 cables can hold six wires for up to three telephone lines, and their nomenclature is technically RJ-11, RJ-14 or RJ-25 depending on the number of lines (see chart below).

RJ-21X, RJ-26X and RJ-27X
For PBX and other telecom connections, RJ-21X, 26X and 27X cables use a 50-pin Centronics-style "Miniature Ribbon" connector to handle 25 telephone lines in one cable.

C and W
The C refers to a desk phone jack, such as RJ-11C, while RJ-11W means a wall-mounted jack. See modular connector.

RJ-11 Connectors
Four-wire RJ-11 connectors (6P4C) are commonly used for telephone handsets and wall outlets. The six-wire variation (6P6C) is used for three telephone lines (see below).

Jack Positions (P)RJ     U.S.                 andType   Wire Pair Colors    Contacts (C)

  RJ-11  Line 1 - red-green     6P2C

  RJ-14  Line 1 - red-green     6P4C
         Line 2 - yellow-black

  RJ-25  Line 1 - red-green     6P6C
         Line 2 - yellow-black
         Line 3 - blue-white
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References in periodicals archive ?
There are two ways: 1.) hook transmitter and output device to slave modular jacks and run extensions (four-lead phone cord with RJ-11 plugs at both ends) to the phone line; or 2.) solder the transmitter and output wire directly to a length of phone line with an RJ-11 plug at the end, and run that to plug into the nearest modular jack on the phone line.
The AR-7211A/B ADSL router features an RJ-11 WAN port for the ADSL connection and an RJ-45 Ethernet LAN port for a wired connection with other network devices.
The new offering integrates a VoIP analog telephone adapter (ATA) and a wireless gateway in a single device, packaged with an AC adapter and RJ-11, RJ-45 and USB cables.
In addition, the device offers voice-testing resources, such as detection of RJ-11 (1, 2 or 3 pair), and the ability to automatically sense and adjust the pairing and pin numbers to the 3-pair USOC standard in cable test, length and tone-generator modes.
I chose to have my wires pass through a small hole drilled in the side of the control box and end with a standard RJ-11 modular connector.
A third version is available for use with GL's Analog Test Set which provides a pair of analog two-wire interfaces (RJ-11) and optional 4-wire cellular phone interfaces for assessing voice quality on analog and wireless systems.
If you do wind up in a one-horse town where they bed the horse right in the hotel, or are traveling abroad, take a phone cord with an RJ-11 connector on one end and four colored wires with under-screw connectors on the other.
The PC CommLink, which is available now for about $10 in retail stores, is said to provide a solution to RJ-11 cable requirements for fax/modern needs.