Three-way switches control an electrical load from two locations. Depending on the location of the light (whether it falls between the switches or after both), the wiring sequence will differ.

In the three-way switch scenario shown above, the fixture is placed between the switches. As before, a three-conductor cable joins the first switch and the fixture, but now a four-conductor cable runs from the fixture to the second switch. The capped off neutral is available in the second switch box should it be needed in the future for a device that demands more complex wiring.
In the three-way switch scenario shown above, the fixture is placed between the switches. As before, a three-conductor cable joins the first switch and the fixture, but now a four-conductor cable runs from the fixture to the second switch. The capped off neutral is available in the second switch box should it be needed in the future for a device that demands more complex wiring.

To learn how to wire a three-way switch when both switches come after the fixture, and sort out other finish wiring head scratchers, visit the JLC Field Guide.

For more about the changes to the 2014 National Electric Code requiring an available neutral, see "Neutral Necessity: Wiring Three-Way Switches."