What is a multiwire branch circuit (MWBC)?
A multiwire branch circuit is two or more ungrounded conductors with a voltage between them, sharing one grounded conductor, where each hot has equal voltage to the neutral. In a house that means a 12-3 cable: black on one leg, red on the opposite leg, sharing the white. Because the hots are on opposite legs of the 120/240 volt service, the neutral carries only the unbalanced current, which is why one white wire can serve two circuits.
The NEC 210.4 rules
Three requirements do most of the work. First, 210.4(B): each multiwire branch circuit needs a means to simultaneously disconnect all ungrounded conductors at the panel, which means a 2-pole breaker or two singles with an identified handle tie. Second, 210.4(A) and 300.13(B): the neutral must be pigtailed at devices, never spliced through a receptacle's screw terminals, so removing a device cannot open the shared neutral. Third, 210.4(D): the conductors must originate from opposite legs, and grouping per 210.4(D) requires the conductors of each MWBC to be grouped together in the panel with a wire tie or similar unless they are in the same cable.
Why the lost neutral is dangerous
If the shared neutral opens while both circuits are loaded, the two circuits become a series loop across 240 volts. Voltage divides by load impedance, so the lightly loaded side can see well over 120 volts and the heavy side well under. Electronics burn up in seconds. This is exactly why the pigtail rule and the handle tie rule exist, and why a loose neutral in an MWBC is the classic cause of half the house going bright while the other half goes dim.
Where MWBCs make sense
Kitchen small appliance circuits, dishwasher and disposal combos, and long homeruns where copper savings matter. Watch two interactions: AFCI and GFCI protection on shared-neutral circuits requires 2-pole devices or careful device selection, and both hots on the same leg is an outright defect, since the neutral then carries the sum of both currents and can overheat on a conductor with no overcurrent device protecting it at that load.