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No doubt you have seen lights (or electric appliances) controlled by two or more switches. Each switch is part of one electric circuit that can turn the light on or off, irrespective of the position of the other switches. You often encounter this system in situations where it is inconvenient to return to the place where you initially turned on a light. For instance, the lighting in hotel rooms needs to be controlled at the entrance, as well as the bed.

How do electricians do this? What you need are 3-way switches to control the lighting. A 3-way switch has two switch positions, connecting either (1) A to B, or (2) A to C (see diagram).

```               •----B                 •----B
/
/
A----•                 A----•
\
\
•----C                 •----C

(3)                   (3)
```

If the circuit requires 3 or more switches to control an electric appliance, you need additional 4-way switches. These switches also have two switch positions and connect either (1) A to D and C to B, or (2) A to B and C to D. (see diagram).

```       A----•     •----B      A----•-----•----B
\   /
\ /
X
/ \
/   \
C----•     •----D      C----•-----•----D

(4)                    (4)
```

A circuit requiring two switches is built up from two 3-way switches as follows:

```        S-----------*-----------|
|
|
•-------•       |
/         \      |
/           \     |
S----•             •----|

•-------•

(3)      (3)
```

Where (S) is the power source, and (*) is a light fixture or electric appliance (currently shown in the ON position).

A circuit requiring three switches has a similar configuration, but an additional 4-way switch is added in between the two 3-way switches:

```        S------------------*------------------|
|
|
•-------•     •-------•       |
\   /         \      |
\ /           \     |
S----•             X             •----|
\           / \
\         /   \
•-------•     •-------•

(3)        (4)           (3)
```

Where (S) is the power source, and (*) is a light fixture or electric appliance (currently shown in the ON position). Additional controls can be built by installing additional 4-way switches in between the two 3-way switches.

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