Originally Posted by
randyc
3-way switches are A/B switches. Power comes in on one end and goes up to the light (switch leg) on the other. As long as both switches are set to A or both to B, the light will be on. The two switches are connected by a 3-conductor cable (black, white, red). Normally the black and red are connected to the switch as travelers and the white is used as a neutral. Sometimes the power and switch leg are physically at the same end. Electricians use one of the conductors to relay either the power or switch leg to the other end and the white becomes a traveler.
You only need to look at the wires connected to the switches. Your dimmer will have power coming in on the black and out to the light or next switch on the red. The red/white is used for communication.