What do the yellow and black wires in a home telephone jack do?

It only takes two wires to connect a phone, but most house wiring contains four wires. Check out this HowStuffWorks podcast to learn why.

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Episode Transcript

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(00:23):
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brain stuff today for details. Hi, I'm Marshall brain with
today's question. What do the yellow and black wires in

(00:46):
a home telephone jack do? In your house? You have
phone jacks, it only takes two wires to connect to 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

(01:09):
houses to handle two phone lines very easily. Many, if
not most, of the line chords 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 two outer wires are omitted when you
bring a second line into your house. This new line

(01:29):
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 those yellow and black wires.
You plug one line cord into the phone and it
splits the two lines inside the phone. Make sure that
that line cord between the jack and the phone actually
contains all four wires for this to work. If you

(01:52):
want to connect something like a fax machine to the
second phone line, you can't just plug it into the wall.
You normally need to buy a little plug and converter
that takes the two outer terminals yellow and black of
the phone jack and brings them to the two center
terminals the red and green. You plug the converter into
the wall and then the fax machine into the converter.

(02:14):
Do you have any ideas or suggestions for this podcast?
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how stuff works dot com. For more on this and
thousands of other topics, go to how stuff works dot com.
Brain Stuff is sponsored by Harry's dot com. Get a
great shaving experience for a fraction of the price. The
quality of the shave is so important that Harry's purchased

(02:35):
the factory that makes their blades. Fifteen dollars gets you
a set that includes a handle, three blades, and shave cream.
Go to Harry's dot com. Use the code brain stuff
to say five dollars off your first purchase.