witches together, toggling either switch changes the state of the load from off to on, or on to off. The switches may be arranged so that they are in the same orientation for off, and contrasting orientations for on.
A "4-way" (intermediate) switch is a purpose built Double Pole, Double Throw (DPDT) switch, internally wired in manufacture to reverse the connections between the input and output and having only four external terminals. This switch has two pairs of "traveler" terminals that it connects either straight through, or crossed over (transposed, or swapped). An intermediate switch can, however, be implemented by adding appropriate external wiring to an ordinary (six terminal) DPDT switch.
By connecting one or more 4-way (intermediate) switches in-line, with 3-way switches at either end, the load can be controlled from three or more locations. Toggling any switch changes the state of the load from off to on, or from on to off.
Number of wires
In this article, when comparing different wiring methods or systems the focus is on the number of wires connecting the switches to each other. Sometimes an additional wire is present in each cable to carry an unswitched neutral or hot wire which passes through the switch mounting box, but is not normally connected to the switch. If the switch has a pilot light or a receptacle, additional hot or neutral connections to the device may be required. Grounding (earthing) wires are not counted.
Switching a load on or off from two locations (for instance, turning a light on or off from either end of a flight of stairs) requires two SPDT switches. There are several arrangements of wiring to achieve this.
In the traveler system, also called the "common" system, the power line (hot, shown in red) is fed into the common terminal of one of the switches; the switches are then connected to each other by a pair of wires called "travelers" (or "strappers" in the UK), and the lamp is connected to the common line of the second switch, as shown.
Using the traveler system, there are four possible combinations of switch positions: two with the light on and two with the light off.
In the simple situation of two switches and a single switched load this system offers no advantage, and in fact has the disadvantage of requiring four wires (including neutral) between "ends" of the installation, compared to three wires in the standard system.
However, in the unusual case in which a switched load is wanted at both ends (e.g. illuminating a long hallway), and an unswitched load (e.g. receptacle) is wanted at both ends as well, this system's four wires saves one wire compared to the standard system, which would require five wires (two travelers, one neutral, one unswitched-hot, one switched-hot) to serve all these loads. However, in this application the system cannot be extended (e.g. with "four way" switches) to offer more than two switch locations.
The Carter system was a method of wiring 3-way switches in the era of early knob-and-tube wiring. This now-obsolete wiring method has been prohibited by the National Electrical Code since 1923, even in new knob-and-tube installations which are still permitted under certain circumstances. This wiring system may still be encountered in older "grandfathered" electrical installations, however.
In the Carter system, the incoming live (energized) and neutral wires were connected to the traveler screws of both 3-way switches, and the lamp was connected between the common screws of the two switches. If both switches were flipped to hot or both were flipped to neutral, the light would remain off; but if they were switched to opposite positions, the light would illuminate. The advantage of this method was that it used just one wire to the light from each switch, having a hot and neutral in both switches.
The major problem with this method is that in one of the four switch combinations the socket around the bulb is electrified at both of its terminals even though the bulb itself is not lit. As the shell may be energized, even with the light switched off, this poses a risk 