Why Is It Called A Phone Jack?

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Why do we call the place where you plug in a telephone a 'jack'? It's kind of strange when you think about it. It doesn't look like a person or the thing you use to prop up a car while you change a tire. It's just a specifically shaped hole. So, how'd it get that name?


What Does Telephone Jack Mean?

A telephone jack is a socket into which a connector that is used to connect a telephone to a building’s wiring is plugged. It acts as a hardware interface between the telephone wiring and the telephone, and is considered the counterpart of the telephone plug. It is usually fixed to the wall or baseboard.


Every country follows a particular standard for the design and use of telephone jacks. The most commonly used telephone jack standard is the RJ-11 modular connector. The telephone jack forms a part of the dial-up systems used by home computers to connect to the Internet.


A telephone jack is also known simply as a phone jack.

 phone jack

The following are the most common types of telephone jacks:


Flush plate — A wall-mount jack that sticks out from the wall, allowing the telephone to be mounted on the wall

Wall mount — A wall-mount jack that allows for plugging in the phone but does not mount it

Baseball jack — A type of jack that consists of small plastic or metal boxes fixed to a baseboard. The jack opens at the bottom edge of the box.


The following are some of the most common telephone jack standards:


RJ-11 — This jack has six conductors and is usually implemented with four conductors. The four wires used are grouped as a red-and-green and a black-and-white colored pairs. Dial-up modems are connected to a network using RJ-11 jacks.

RJ-14 — This jack also has four wires, which are used for two phone lines.

RJ-45 — This is a single-line jack used in digital data transmission, with a data rate of up to 19.3 Kbps.


Elephone wiring


It only takes two wires to connect a phone, but most house wiring contains four wires. The red and green wires are on the two center terminals of a normal phone jack, and the black and yellow wires are on the two outer terminals. This arrangement allows most normal houses to handle two phone lines very easily.


Many (if not most) line cords that you receive when you purchase a phone (to go between the wall jack and the phone) only have the two center wires. To save money, the outer two are omitted.

 phone jack

When you bring a second phone line into the house, this new line uses the yellow and black wires. If you have a two-line phone, then the phone is normally ready to accept the second line automatically on the yellow and black wires. You plug one line cord into the phone and it splits the two lines inside the phone. Make sure that the line cord between the jack and the phone actually contains all four wires!


If you want to connect something like a fax machine on the second phone line, you cannot just plug it into the wall. You normally need to buy a little plug-in converter that takes the outer two terminals (yellow/black) of the phone jack and brings them to the center two terminals (red/green). You plug the converter into the wall and the fax machine into the converter.


Because of the proliferation of fax machines, modems, answering machines and other phone-related devices, many new houses are wired so that they can handle up to six phone lines. All six lines are present at each individual jack, and you decide which two the jack will carry by connecting them to the jack's red/green or yellow/black wires.


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