One wire comes in from the power source, and one wire goes out to the light(s). A 3-way switch adds two terminals for travelers, which connect two 3-way switches together.

That way, you’ll be sure to reconnect the wires correctly to the new switch. If you can’t kill the electricity, the switch box might be connected to a cable from a different circuit.

There are two types of light switches, says Donnelly: a single-pole light switch, which has two wires, and a three-way light switch with three wires that usually look identical.