The difference between a standard switch and a three way switch is one additional terminal,or connection. A 3 way switch has 2 terminals that are either silver or brass coloured and one terminal that is dark coloured. The silver or brass terminals are called travellers or messengers and the dark coloured terminal is called the common. If all three terminals appear similar in colour, the common terminal will be identified by the word "common" stamped on the back of the switch.
The arrangement of these terminals on the switch can vary depending upon the manufacturer, so check carefully to ensure proper identification of the common terminal. In a 3 way switch circuit there will be two 3 way switches and a light or series of lights that are controlled by the switches. As each switch can turn the lights on or off,there needs to be a means by which one switch can communicate this back to the other switch. This is the function of the two travellers or messengers and you will notice in the wiring diagrams that regardless of the configuration, the 2 travellers or messenger terminals on each switch are always interconnected (trace the red and white conductors of the 3 conductor wire).
As the travellers or messenger terminals are always interconnected, the common terminal is the only one left. To understand this terminal, consider for a moment a standard switch that has only 2 terminals. When wiring a standard switch the incoming power feed (hot) conductor is connected to one terminal (usually bottom right) and the outgoing power feed to the lights is connected to the other terminal (usually top right). This is the purpose of the common terminals in a 3 way switch circuit, on one of the two 3 way switches the incoming power feed (hot) conductor is connected to its common terminal and on the other of the two 3 way switches the outgoing feed to the lights is connected to its common terminal.
It is important to note that the white conductor in the 3 conductor wire (red, black and white) connected between the two switches is not a neutral. Depending on the switches on/off position either the red or white conductor will be hot.
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