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. 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. 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
