three-way switch
A type of light switch that can be arranged in pairs to control lights from two locations, as at either end of a stairway or long hallway; a double-throw switch. (UK): two-way switch.
three-way switch: control one light from two places
A three-way switch is an electrical switch that toggles a load (usually a light fixture) from either of two remote locations. The name refers to the three terminals on the switch: two for incoming hot conductor positions and one for the outgoing load conductor. When you flip the switch at one location, the light changes state regardless of the position of the switch at the other location. This is distinct from a standard single-pole switch, which has only two terminals and controls a load from one location only.
Three-way switches are wired in pairs, always. The circuit requires a three-wire or four-wire cable connecting the two switches; in older installations, this might be a cloth-wrapped cable, while modern code requires individual insulated conductors in a conduit or cable sheath. The neutral and ground conductors run continuously through both switches, but the hot conductor connects to one of the switch terminals. Between the switches, two conductors called travelers form the loop that determines whether current flows to the fixture. Each switch changes which traveler the hot conductor flows through.
Variants and failure modes
A four-way switch can be inserted between two three-way switches to control a light from three or more locations. The four-way switch has four terminals: two for incoming travelers and two for outgoing travelers. It essentially reverses the circuit state each time it is toggled. Dimmer switches that function as three-way switches do exist but are less common and must be matched to compatible companions. Wiring errors are the most frequent fault: reversed travelers, a broken traveler conductor, or a neutral disconnected at one location will cause the switches to work erratically or not at all.
The terminology is regional. In the UK and Commonwealth countries, this switch is called a two-way switch because it has two positions (up and down), and the circuit has two control points. In North American usage, the three-way name stuck because of the three terminals. This historical naming difference continues to cause confusion in trade documentation and when ordering parts across borders.
Three-way switches are a standard requirement in residential and commercial wiring codes for controlling lights at stairwells, hallway ends, large rooms with multiple entries, and outdoor fixtures with switches near different doors. The switches themselves are inexpensive and reliable; the cost lies in running the extra traveler conductors and in the more complex wiring labor. Loose or corroded connections at the terminals are the primary maintenance issue in older installations, while in new work, incorrect wire gauging or failure to support cables properly can lead to early failure.