How does a three way switch work?...
A three-way switch has three screw terminals. One screw terminal is colored darker than the other two. It's called the common terminal.The other two screws are called traveler terminals. Two of these switches are needed to turn a light off and on from two different locations. The black wire from the light usually goes to the common terminal on one of the switches, and the black wire from the 120 volt feed goes to the common terminal of the other switch. The white wire ( neutral ) is fed from the 120 volt feed through the two three way switches and on to the white wire of the light. A three conductor (black red white) cable is placed in between the two three way switches. The black and red wires are connected to the traveler terminals of each switch and the white wire is connected together at each switch from the feed to the light. To turn lights on from more than two areas, four way switches are placed in between the travelers of the two three way switches. Three and four way switches do not have on and off markings on the toggle as do regular switches. These switches can be confusing, and should be installed by a qualified person.
Four Way Switch
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