Three way switches are used to have two switches turn on light(s). They can do that because there is always power to both switches even if one of the switches is turned off. This article teaches you how to wire a three way switch.
brief how to
A three way switch has 4 terminals:
The First box: The First box has the power come into it with a 2 wire cable, and a 3 wire cable going out if it to the second box and switch.
The Second: The second box has the 3 wire cable coming in from the first box and a 2 wire cable going out to the light.
The Traveler: The 3 wire cable that goes between boxes is called the "Traveler". The 3 wire cable allows power to go to the second switch even when the first switch is turned off.
Bring the two wire power cable, from the breaker panel, into the first switch box and attach the black wire to the common terminal on the switch. (the common terminal is usually black in color)
Bring the three wire "traveler" cable into the first switch box and attach the black and red wires to the traveler terminals, (the two terminals that are the same color)
Attach the white wires from the power cable and traveler cable to each other with a wire nut.
Attach the ground wires to each other with a grounding wire nut and then attach the ground to the switch.
Push the wires into the box and screw the switch to the box.
The 3 wire traveler cable goes from the first box to the second switch box. To hook up the second switch you will hook the black and red wires from the 3 wire traveler cable to the terminals on the switch in the same position, sides, as on the first switch.
Run 2 wire cable goes from the second box to the lights. Attach the black wire that goes to the light to the common terminal. (the power comes into the first switch on the common terminal and goes out of the seconde switch to the light(s) on the common terminal).
After the connections are made put the wires into the box and screw the switch to the box. Use wire nuts to
Attach the light to the 2 wire cable. Black to Black, White to White, and Ground to Ground. Push the wires into the ceiling box and attach the light fixture to it.
Turn the breaker on and test your 3 way light switches.
Attach the switch to the box and test it.