|Three-way switches can be a little confusing because,
unlike a standard switch, they have three screw terminals
and do not have ON-OFF markings. Three-way switches are
always installed in pairs and are used to control a set of
lights from two separate locations.
One of the screw terminals is darker than the others.
This is the common screw
terminal. Before disconnecting a three-way switch, always
label the wire that is connected to the common screw
terminal. It must be reconnected to the common screw
terminal on the new switch.
The two lighter-colored screw terminals are called the
traveler screw terminals. They are interchangeable so
there is no need to label the wires attached to them.