Split phase shared neutral (North American Wiring)
In split phase house wiring, for example, a duplex receptacle in a
kitchen is typically connected with a cable that has three conductors,
in addition to ground. The three conductors are usually colored red,
black, and white. The white serves as a common neutral, while the red
and black each feed, separately, the top and bottom hot sides of the
receptacle. Typically such receptacles are supplied from a ganged
breaker, i.e. a breaker in which the handles are tied together for a
common trip, so that if one kitchen appliance malfunctions and pops
the breaker, the other side of the duplex receptacle will be shut off
as well. This is called a multiwire circuit.
These types of circuits must use a double pole circuit breaker
While the NEC recognizes this configuration, there is open debate as to weather or not this is a safe practice. For more information see Understanding the Dangers of Multiwire Branch Circuits
Multiwire circuits for 3 and 4-way switches
Switch legs, also called travelers must now be in a different color to be easily identified. A few years ago the white wire could be used as a traveler but code changed that to lessen the confusion of identifying the traveler. Now instead of running a piece of 14/2W/GRD or 12/2W/GRD for 3-way you have to use 14/3W/GRD or 12/3W/GRD. Same goes for 4 Way's also.