Permanent live is taken from one non-common terminal and switched live is taken from the other. The second system, which is the norm in the UK nowadays and which Americans call 'California 3 way' or 'coast 3 way', is to connect all three terminals of the switches together. If the two switches are switched to the same traveller the light is on, if they are switched to different travellers the light is off. The other terminals of the switches are connected together by the 'travellers'. from the common terminal of the other switch. The permanent live is fed into the common terminal of one switch, the live to the light is fed. The first system and the easiest to understand is the system described in Dave's and Harper's answers. There are two main wiring arrangements that can be used to control a light from multiple locations. It has three terminals, one of which is a 'common' which depending on the position of the switch connects to one of the other two terminals. What American electricians refer to as a '3 way switch' and British electricians refer to as a '2 way switch'* is what electronics guys would call a SPDT switch. Normally (excluding stuff like intelligent dimmers), the switches are just switches. Man, I really shoulda marked those travelers with yellow tape. Most homes are wired in cable, and so you get a box with a 3-way and other stuff going on, and you have 9 wires with every one being black, white or red. Doing so makes working with 3-ways easy failing to do so makes it a nightmare. I like to use yellow wire or mark the wires with yellow tape. Notably, physical position of the terminals on the switch is meaningless every switch is different. These live on the 2 'traveler' or non-'common' terminals, and in North American practice are colored brass.

The usual place people get into trouble with 3-ways in household wiring is failing to properly identify the travelers. But you can make one out of a DPDT by wiring an 'X' connecting the non-common terminals, then attaching the wires to the one side of the 'X' and of course the commons. This is done by a reversing switch.Ī reversing switch isn't quite a DPDT. However in the middle, you need to exchange the travelers. Any 3/4 way switch alternates which one is hot or not. Here is how they work it is simplicity itself.

3 way switch circuit