Use this tag for questions about Multi-Wire Branch Circuits, sometimes called "Edison circuits".

A multi-wire branch circuit is used to conserve wire in North American wiring when dual (line to line and line to ground) utilization voltages are present, such as in the 120/240V split-phase system used in residences or Wye type three phase systems (120/208Y, 277/480Y) found in commercial work.

These require a common disconnecting means (either a pullout disconnect with multiple fuses, multiple single pole breakers linked by handle ties, or a multi-pole breaker), and the neutral must be pigtailed to wiring devices so that removing a device does not unlink the neutral. They are most commonly used for kitchen receptacles (small appliance branch circuits), despite having compatibility issues with and most protection, but may be used for other things as well.