Most of us take for granted the ability to turn a light fixture on or off from the bottom or top of the stairs, or from two different entrances to a room. But a wall switch that allows this bit of magic is not a standard switch. It is called a 3-way switch, and it is wired quite differently from standard—or single-pole—switches.
3-Way Switching Basics
The thing to remember about 3-way switch wiring is that 3-way switches are always used in pairs. In other words, if a circuit has 3-way switches, it always has two of them. If there are other switches on the same circuit, these must be 4-way switches; 3-way switches are not used on the same circuits with standard single-pole switches. A 4-way switch is similar to a 3-way and works in conjunction with a pair of 3-ways to provide switching control from three or more locations.
How to Identify a 3-Way Switch
When viewed from the front, a standard single-pole toggle switch has "on/off" markings next to (or above and below) the toggle. A 3-way switch has no "on" or "off" markings because the toggle can turn the light on or off in either position, and this depends on the position of the other switch's toggle. A 4-way switch also has no "on/off" markings, for the same reason.
A more positive way to identify a 3-way switch is to look at the body of the switch and count the number of screw terminals: a 3-way switch has three terminal screws plus a ground screw. Two of the terminals are a light color—bronze- or copper-colored—and are called travelers. The single dark-colored screw is known as the common terminal. The ground screw is usually green.
The arrangement of these screws varies depending on the switch manufacturer. On some 3-way switches, the two traveler screws are on one side of the switch body, with the common screw isolated on the other side. With other switches, the traveler screws are paired on opposite sides of the switch, with the common screw located on the opposite end of the switch body.
Wiring for a 3-Way Switch
The proper wiring for a 3-way switch depends on where the switch falls in the circuit. Remember that 3-way switches always appear in pairs. The two switches can come before or after the light fixture in the circuit, or you can have one switch on each side, with the fixture in the middle.
The most important wire to get right is the one connected to each switch's common screw terminal. This is the "hot" wire (usually colored black, but not always), and it brings the power from the source and delivers it to the first switch and then from the second switch to the light fixture. Depending on where the switch falls in the circuit, the black wire will either be delivering power to the first switch from the power source, or it will be delivering power onward from the second switch to the light fixture (or to the fixture then the second switch, if the fixture is in the middle).
The two traveler terminals are used to connect the "traveler" wires. The traveler wires run between the two 3-way switches, offering two potential pathways to complete the circuit and send power onward to the light fixture. It makes no difference which traveler wire goes to which traveler terminal on the switch; the traveler terminals are interchangeable.
Note that the terminal wires also are considered "hot" because they carry power when the switches are on. As a safety measure, if a terminal wire is white (which is usually a "neutral" wire), it should be labeled with a band of black or red tape near the switch, indicating it is a hot (not a neutral) wire.
The other wires in a circuit with 3-way switches are the neutral (usually white) and ground (usually bare copper or green). The neutral wire bypasses both 3-way switches but connects to the light fixture. The ground connects to the ground terminal on each switch and to the light fixture.
Easiest Way to Replace a 3-Way Switch
If you're simply swapping an old 3-way switch for a new one, use this simple trick to get the wiring right: Before disconnecting any wires on the old switch, find the wire attached to the common screw terminal and label it with a piece of tape. Then, you can disconnect all three wires from the switch, as well as the ground wire. Because the other two wires (the travelers) are interchangeable, it doesn't matter which of the traveler screws you attach them to—there's no need to label them.
Tutorial: How 3-way and 4-way switch circuits work. Wake Forest University.