The image shown here is a simple illustration of the basic principle of operation of a common mode choke. Common mode currents i1 and i2, flowing in the same direction through each of the choke windings, creates equal and in-phase magnetic fields which add together. This results in the choke presenting a high impedance to the common mode signal, which passes through the choke heavily attenuated. The actual attenuation or common mode rejection depends on the relative magnitudes of the choke impedance.