Multiway switching

A common application is in lighting, where it allows the control of lamps from multiple locations, for example in a hallway, stairwell, or large room.

When the load is controlled from only two points, single pole, double throw (SPDT) switches are used.

In alternative designs, low-voltage relay or electronic controls can be used to switch electrical loads, sometimes without the extra power wires.

This switch has two pairs of "traveler" terminals that it connects either straight through, or crossed over (transposed, or swapped).

This now-obsolete wiring method has been prohibited by the USA National Electrical Code since 1923,[2] even in new knob-and-tube installations which are still permitted under certain circumstances.

As the shell may be energized, even with the light switched off, this poses a risk of electrical shock when changing the bulb.

This method is therefore prohibited in modern building wiring where Edison screw based lamps are used.

Systems based on relays with low-voltage control circuits permit switching the power to lighting loads from an arbitrary number of locations.

For each load, a latching relay is used that mechanically maintains its on- or off-state, even if power to the building is interrupted.

Any number of additional rocker switches can be wired in parallel, as needed in multiple locations.

After an initial burst of popularity in the 1960s, residential use of such relay-based low voltage systems has become rare.