In the electrical circuit of a circuit board, the current flowing out from a certain part reaches another circuit through the load, and returns to the origin via a different route on the circuit board. (In many cases, the return route is the ground plane of the circuit board.) This type of flow is called differential mode (or normal mode).

Another conduction route also exists, though not as a clear-cut wire. A tiny amount of stray capacitance is generated between the wires on the circuit board and the reference ground surface, creating a conduction route where the capacitance flows commonly through all wires on the circuit board and returns in the opposite direction along the reference ground surface. This route is called common mode.

Although the stray capacitance between the wires and the reference ground surface is quite small, impedance drops as the signal frequency rises even with the tiny amount of stray capacitance, so that the common mode current flows more easily. Normally, the common mode current is not actively sent through the electrical circuit, but if the ground of a power supply circuit or driver IC vibrates, the entire circuit it drives will vibrate, resulting in common mode noise. If a cable is externally connected to the circuit, common mode current will also flow through the cable. As it will have an electrical potential with respect to the ground, the current will be released as noise radio waves.