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You may also be interested to see how a four way switch works
A 3-way switch will usually have 4 screws. Most older 3 way switches only have 3 screws. The difference between the older and the newer switch is, the ground screw. Code changes over the years have required the addition of the ground screw for safety reasons. However, it is not needed for the correct operation of the 3 way switch. It is needed for the safe operation of this switch or any other type of switching device. On some switches the colors of the screw terminals are different to differentiate between the common and traveler screws. Some switches are made differently from others so the colors used may differ between manufacturers. The common screw will nearly always be a different color from the traveler screws.