When this interference is propagated, it can either enter or leave a system by symetrical or asymetrical propagation. Symetrical propagation is when the noise signal enters and leaves the AC mains in an alternating fashion (differentail mode). Symetrical propagation is resolved via the application of X capacitors, with the capacitor placed directly across the AC mains. Conversly, asymetrical propagation enters and leaves both sides of the AC mains at the same time, and propagates via ground. This is resolved by the application of Y capacitors, placed on both sides of the AC mains to the system chassis (ground). In both modes, the capacitor functions as a low impedance path for high frequency signals, and hence, a filter for those signals.