Common mode chokes (CMC’s) are most often used as filters on the input line for switching power supply circuits. They are wound on a high permeability core, most often toroidal or UU cores, with two windings. The windings are mirror wound, with one on the line side and one on the neutral side of the input AC.
Under normal operation, the current through each winding is equal and opposite. Because of the mirror winding, the flux in the core cancels out. This allows the CMC to present a very low impedance to the input AC line.